New --web-define feature breaks existing html templates and cannot be disabled

### Steps to reproduce

1. Create a flutter web project
2. Include syntax such as `{{else}}` for HTML templating in the index.html file
3. Attempt to build for web

### Expected results

The flutter web project builds or runs successfully, with the html template syntax existing as it should.

### Actual results

Flutter overreaches and tries to template everything and it's (seemingly) not possible to disable it, prompting the following error
```sh
Target web_templated_files failed: Error: Missing web-define variable: flutter_commit_hash

Please provide the missing variable using:
flutter run --web-define=flutter_commit_hash=VALUE
or
flutter build web --web-define=flutter_commit_hash=VALUE
```

In the example I provided (extracted from our production work codebase), we have `<script src="flutter_bootstrap.js?v={{flutter_commit_hash}}" async></script>`. This will later be replaced in a GitHub actions CI and is intended for cache busting. There is no point in replacing this locally as caching doesn't apply there, and obviously there would be no commit hash. Thusly it's inserted in the actions runner. 

While this feature is useful for GitHub actions, **there are many situations in which it completely breaks our previously-fine production codebase because it forces all of these to be defined ahead of time with no clear way of disabling the check**. There are many reasons and situations where it's desirable to include html templates that we don't want Flutter to parse, and this new feature prevents existing code from working.

This might seem tenuous (why not just replace it with some fake data value), but some HTML templates are not meant to be replaced with data at all in the first place. For example, we have in our codebase (slightly modified for brevity but still valid):
```html
{{if .Name}}
<title>{{.Name}} | EpicHire</title>
jobs{{end}}">
{{else}}
<title>Epic Hire</title>
{{end}}
```
We have this in our codebase because we run Flutter in front of a webserver for SEO. The HTML template engine we use needs these when parsing to determine what to show. This breaks because `{{else}}` and `{{end}}` are seen as Flutter variables.

This was caused by [#175805](https://github.com/flutter/flutter/pull/175805). 

In that PR the following is said
```
Error handling

If a variable is missing, the tool provides clear feedback:

Missing web-define variable: API_URL

Please provide the missing variable using:
flutter run --web-define=API_URL=VALUE
or
flutter build web --web-define=API_URL=VALUE
```
I believe this is the wrong approach as it breaks both existing code, and otherwise entirely valid setups for HTML templates.

### Code sample

<details open><summary>Code sample</summary>

```html
<!DOCTYPE html>
<html>

<head>
  <base href="$FLUTTER_BASE_HREF">

  <meta charset="UTF-8">
  <meta content="IE=Edge" http-equiv="X-UA-Compatible">
  <meta name="description" content="A new Flutter project.">

  <meta name="mobile-web-app-capable" content="yes">
  <meta name="apple-mobile-web-app-status-bar-style" content="black">
  <meta name="apple-mobile-web-app-title" content="bug">
  <link rel="apple-touch-icon" href="icons/Icon-192.png">

  <link rel="icon" type="image/png" href="favicon.png" />

  <title>bug</title>
  <link rel="manifest" href="manifest.json">
</head>

<body>
  <script src="flutter_bootstrap.js?v={{flutter_commit_hash}}" async></script>
</body>

</html>
```

</details>


### Screenshots or Video

<details open>
<summary>Screenshots / Video demonstration</summary>

[Upload media here]

</details>


### Logs

<details open><summary>Logs</summary>

```console
flutter build web --release --wasm --base-href / --release --source-maps --no-tree-shake-icons --dart-define=EPICHIRE_ENV=PROD
Resolving dependencies... 
Downloading packages... 
  _fe_analyzer_shared 91.0.0 (94.0.0 available)
  _flutterfire_internals 1.3.65 (1.3.66 available)
  analysis_server_plugin 0.3.4 (0.3.8 available)
! analyzer 8.4.1 (overridden) (10.0.2 available)
  analyzer_buffer 0.1.11 (0.3.1 available)
  analyzer_plugin 0.13.11 (0.14.2 available)
  build 4.0.3 (4.0.4 available)
  build_runner 2.10.4 (2.11.0 available)
  built_value 8.12.1 (8.12.3 available)
  carousel_slider 5.1.1 (5.1.2 available)
  code_builder 4.11.0 (4.11.1 available)
  cross_file 0.3.5+1 (0.3.5+2 available)
  custom_lint_core 0.8.1 (0.8.2 available)
! custom_lint_visitor 1.0.0+8.4.0 (overridden) (1.0.0+9.0.0 available)
  dart_helper_utils 5.4.0 (6.0.0 available)
  dart_mappable_builder 4.6.3 (4.6.4 available)
  dart_pubspec_licenses 3.0.14 (3.0.15 available)
  dart_style 3.1.3 (3.1.5 available)
  dbus 0.7.11 (0.7.12 available)
! device_info_plus 12.3.0 (overridden)
  dio 5.9.0 (5.9.1 available)
  emoji_picker_flutter 4.3.0 (4.4.0 available)
  equatable 2.0.7 (2.0.8 available)
  ffi 2.1.4 (2.2.0 available)
  file_picker 10.3.7 (10.3.10 available)
  firebase_analytics 12.0.4 (12.1.2 available)
  firebase_analytics_platform_interface 5.0.4 (5.0.6 available)
  firebase_analytics_web 0.6.1 (0.6.1+2 available)
  firebase_core 4.3.0 (4.4.0 available)
  firebase_core_web 3.3.1 (3.4.0 available)
  firebase_crashlytics 5.0.4 (5.0.7 available)
  firebase_crashlytics_platform_interface 3.8.15 (3.8.17 available)
  firebase_messaging 16.1.0 (16.1.1 available)
  firebase_messaging_platform_interface 4.7.5 (4.7.6 available)
  firebase_messaging_web 4.1.1 (4.1.2 available)
  flutter_form_builder 10.2.0 (10.3.0+1 available)
  flutter_local_notifications 19.5.0 (20.0.0 available)
  flutter_local_notifications_linux 6.0.0 (7.0.0 available)
  flutter_local_notifications_platform_interface 9.1.0 (10.0.0 available)
  flutter_local_notifications_windows 1.0.3 (2.0.0 available)
  flutter_markdown 0.7.7+1 (discontinued replaced by flutter_markdown_plus)
  flutter_riverpod 3.1.0 (3.2.1 available)
  flutter_web_auth_2 4.1.0 (5.0.1 available)
  flutter_web_auth_2_platform_interface 4.1.0 (5.0.0 available)
  form_builder_validators 11.2.0 (11.3.0 available)
  go_router 17.0.1 (17.1.0 available)
  google_fonts 6.3.3 (8.0.1 available)
  google_sign_in_android 7.2.4 (7.2.7 available)
  google_sign_in_ios 6.2.3 (6.2.5 available)
  haptic_feedback 0.5.1+1 (0.6.4+3 available)
  hive_ce 2.15.1 (2.19.3 available)
  hive_ce_flutter 2.3.3 (2.3.4 available)
  image 4.5.4 (4.7.2 available)
  image_picker_android 0.8.13+7 (0.8.13+13 available)
  image_picker_for_web 3.1.0 (3.1.1 available)
  image_picker_ios 0.8.13+1 (0.8.13+6 available)
  isolate_channel 0.2.2+1 (0.6.1 available)
  json_annotation 4.9.0 (4.10.0 available)
  kalender 0.14.2 (0.14.3 available)
  keyboard_height_plugin 0.1.5 (0.2.0 available)
  lints 6.0.0 (6.1.0 available)
  media_kit 1.2.3 (1.2.6 available)
  meta 1.17.0 (1.18.1 available)
  metalink 1.0.4 (2.0.0 available)
  metalink_flutter 1.0.2 (2.0.1 available)
  mixpanel_flutter 2.4.4 (2.5.0 available)
  mobile_scanner 7.1.3 (7.1.4 available)
  mockito 5.6.1 (5.6.3 available)
  pana 0.22.24 (0.23.9 available)
  path_provider_foundation 2.5.1 (2.6.0 available)
  pdfium_dart 0.1.2 (0.1.3 available)
  pdfium_flutter 0.1.6 (0.1.9 available)
  pdfrx 2.2.12 (2.2.24 available)
  pdfrx_engine 0.3.3 (0.3.9 available)
  photo_manager 3.7.1 (3.8.3 available)
  pinput 6.0.1 (6.0.2 available)
  riverpod 3.1.0 (3.2.1 available)
  riverpod_analyzer_utils 1.0.0-dev.8 (1.0.0-dev.9 available)
  riverpod_annotation 4.0.0 (4.0.2 available)
  riverpod_generator 4.0.0+1 (4.0.3 available)
  riverpod_lint 3.1.0 (3.1.3 available)
  rrule 0.2.17 (0.2.18 available)
  safe_local_storage 2.0.1 (2.0.3 available)
  shared_preferences_android 2.4.18 (2.4.20 available)
  simple_gradient_text 1.3.0 (1.4.0 available)
  source_gen 4.1.1 (4.2.0 available)
  source_span 1.10.1 (1.10.2 available)
! super_clipboard 0.9.1 (overridden)
  talker 5.0.2 (5.1.13 available)
  talker_logger 5.0.2 (5.1.13 available)
  tar 2.0.1 (2.0.2 available)
  test 1.28.0 (1.29.0 available)
  test_api 0.7.8 (0.7.9 available)
  test_core 0.6.14 (0.6.15 available)
! timezone 0.10.1 (overridden) (0.11.0 available)
  url_launcher_ios 6.3.6 (6.4.0 available)
  url_launcher_web 2.4.1 (2.4.2 available)
  vector_graphics_compiler 1.1.19 (1.1.20 available)
  video_player_android 2.8.22 (2.9.1 available)
  video_player_avfoundation 2.8.8 (2.9.2 available)
  watcher 1.2.0 (1.2.1 available)
! web_socket_channel 3.0.3 (overridden)
Got dependencies!
1 package is discontinued.
98 packages have newer versions incompatible with dependency constraints.
Try `flutter pub outdated` for more information.

┌─ New feature ────────────────────────────────────────────────────────────────────────────┐
│   WebAssembly compilation is new. Understand the details before deploying to production. │
│   See https://flutter.dev/to/wasm for more information.                                  │
└──────────────────────────────────────────────────────────────────────────────────────────┘
Target web_templated_files failed: Error: Missing web-define variable: flutter_commit_hash

Please provide the missing variable using:
flutter run --web-define=flutter_commit_hash=VALUE
or
flutter build web --web-define=flutter_commit_hash=VALUE
#0      throwToolExit (package:flutter_tools/src/base/common.dart:34:3)
#1      WebTemplate._applyVariableSubstitutions (package:flutter_tools/src/web_template.dart:191:5)
#2      WebTemplate.withSubstitutions (package:flutter_tools/src/web_template.dart:136:18)
#3      WebTemplatedFiles.build (package:flutter_tools/src/build_system/targets/web.dart:735:55)
<asynchronous suspension>
#4      _BuildInstance._invokeInternal (package:flutter_tools/src/build_system/build_system.dart:937:9)
<asynchronous suspension>
#5      Future.wait.<anonymous closure> (dart:async/future.dart:546:21)
<asynchronous suspension>
#6      _BuildInstance.invokeTarget (package:flutter_tools/src/build_system/build_system.dart:875:32)
<asynchronous suspension>
#7      Future.wait.<anonymous closure> (dart:async/future.dart:546:21)
<asynchronous suspension>
#8      _BuildInstance.invokeTarget (package:flutter_tools/src/build_system/build_system.dart:875:32)
<asynchronous suspension>
#9      FlutterBuildSystem.build (package:flutter_tools/src/build_system/build_system.dart:684:16)
<asynchronous suspension>
#10     WebBuilder.buildWeb (package:flutter_tools/src/web/compile.dart:101:34)
<asynchronous suspension>
#11     BuildWebCommand.runCommand (package:flutter_tools/src/commands/build_web.dart:299:5)
<asynchronous suspension>
#12     FlutterCommand.run.<anonymous closure> (package:flutter_tools/src/runner/flutter_command.dart:1590:27)
<asynchronous suspension>
#13     AppContext.run.<anonymous closure> (package:flutter_tools/src/base/context.dart:154:19)
<asynchronous suspension>
#14     CommandRunner.runCommand (package:args/command_runner.dart:212:13)
<asynchronous suspension>
#15     FlutterCommandRunner.runCommand.<anonymous closure> (package:flutter_tools/src/runner/flutter_command_runner.dart:496:9)
<asynchronous suspension>
#16     AppContext.run.<anonymous closure> (package:flutter_tools/src/base/context.dart:154:19)
<asynchronous suspension>
#17     FlutterCommandRunner.runCommand (package:flutter_tools/src/runner/flutter_command_runner.dart:431:5)
<asynchronous suspension>
#18     FlutterCommandRunner.run.<anonymous closure> (package:flutter_tools/src/runner/flutter_command_runner.dart:307:33)
<asynchronous suspension>
#19     run.<anonymous closure>.<anonymous closure> (package:flutter_tools/runner.dart:104:11)
<asynchronous suspension>
#20     AppContext.run.<anonymous closure> (package:flutter_tools/src/base/context.dart:154:19)
<asynchronous suspension>
#21     main (package:flutter_tools/executable.dart:103:3)
<asynchronous suspension>

Compiling lib/main.dart for the Web...                             82.1s
Error: Failed to compile application for the Web.
```

</details>


### Flutter Doctor output

<details open><summary>Doctor output</summary>

```console
flutter doctor --verbose
[✓] Flutter (Channel beta, 3.41.0-0.3.pre, on macOS 26.3 25D5112c darwin-arm64, locale en-US) [332ms]
    • Flutter version 3.41.0-0.3.pre on channel beta at /Users/eric/Documents/development/libraries/flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision af179acb13 (5 days ago), 2026-02-06 15:55:20 -0800
    • Engine revision 492bf6df86
    • Dart version 3.11.0 (build 3.11.0-296.5.beta)
    • DevTools version 2.54.1
    • Feature flags: enable-web, enable-linux-desktop, enable-macos-desktop, enable-windows-desktop, enable-android, enable-ios,
      cli-animations, enable-native-assets, enable-swift-package-manager, omit-legacy-version-file, enable-lldb-debugging,
      enable-uiscene-migration

[!] Android toolchain - develop for Android devices (Android SDK version 36.0.0) [2.3s]
    • Android SDK at /Users/eric/Library/Android/sdk
    • Emulator version 35.6.11.0 (build_id 13610412) (CL:N/A)
    • Platform android-36, build-tools 36.0.0
    • ANDROID_HOME = /Users/eric/Library/Android/sdk
    • ANDROID_SDK_ROOT = /Users/eric/Library/Android/sdk
    • Java binary at: /Applications/Android Studio.app/Contents/jbr/Contents/Home/bin/java
      This is the JDK bundled with the latest Android Studio installation on this machine.
      To manually set the JDK path, use: `flutter config --jdk-dir="path/to/jdk"`.
    • Java version OpenJDK Runtime Environment (build 21.0.6+-13391695-b895.109)
    ! Some Android licenses not accepted. To resolve this, run: flutter doctor --android-licenses

[✓] Xcode - develop for iOS and macOS (Xcode 26.2) [1,003ms]
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Build 17C52
    • CocoaPods version 1.16.2

[✓] Chrome - develop for the web [4ms]
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Connected device (3 available) [6.5s]
    • Eric Apostal’s iPhone (wireless) (mobile) • 00008101-00011DE03E22001E • ios            • iOS 26.2.1 23C71
    • macOS (desktop)                           • macos                     • darwin-arm64   • macOS 26.3 25D5112c darwin-arm64
    • Chrome (web)                              • chrome                    • web-javascript • Google Chrome 144.0.7559.133
    ! Error: Browsing on the local area network for Kevin’s iPhone 12. Ensure the device is unlocked and attached with a cable or
      associated with the same local area network as this Mac.
      The device must be opted into Developer Mode to connect wirelessly. (code -27)

[✓] Network resources [144ms]
    • All expected network resources are available.

```

</details>


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New --web-define feature breaks existing html templates and cannot be disabled #182243

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New --web-define feature breaks existing html templates and cannot be disabled #182243

Description

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Expected results

Actual results

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