[flutter_lints] version 2.0 (flutter#1165)

[pigeon] Fix java crash for nullable nested type (flutter#1192)

[metrics_center] await future in metrics_center (flutter#1202)

[pigeon] implements background platform channels support (flutter#1022)

* [pigeon] added support for background platform channels

* added objc support

* updated the docstring

* added java support

* added error if taskqueues are used with flutter apis

* added compilation test for java

* added ios compilation test

* updated readme and version

* stuart feedback

[pigeon] implements nullable arguments (flutter#895)

* [pigeon] implemented nullable arguements

* implemented dart

* implemented java

* objc implementation

* added generated unit tests

* fixed some objc tests

* added a dart platform test

* added ios platform tests

* added platform test for android

* fixed rebase

* updated pubspec

* stuart feedback

* updated readme

* updated nullability of async nonnull parameters for objc

* updated version to 2.0.0

* stuart feedback

[go_router] Applies flutter package lints (flutter#1017)

[go_router] Keep params in nested routes (flutter#975)

[pigeon] implements nullable return types (flutter#849)

[go_router] adding go_router (flutter#884)

[web_benchmarks] Make package compatible with Chromium v89+ (flutter#518

)

* Added new names for events:
  * isBeginFrame: "WebFrameWidgetImpl::BeginMainFrame"
  * isUpdateAllLifecyclePhases: "WebFrameWidgetImpl::UpdateLifecycle"

* Testing:
  * Add browser.dart unit tests and testing information.
  * Add test matrix to run the benchmark in Chromium 84 and 98.

Co-authored-by: David Iglesias Teixeira <[email protected]>