[flutter_image] Add custom request headers. (flutter#374)

This PR adds custom request headers to address issue flutter/flutter#19532.

[cross_file] Improve dart:io saveTo (flutter#404)

Changes the `saveTo` method for the dart:io-based implementation to copy
path-based instances rather than read them into memory and writing them
out again. This makes it *much* more efficient, especially for large
files.

Also adjusts the unit tests to write to a temporary directory rather
than in-tree, since they are using the actual filesystem.

Fixes flutter/flutter#83665

[flutter_lints] Fix readme (flutter#394)

[flutter_template_images] Support the new skeleton app template (flut…

…ter#366)

This is the flutter_template_images part of flutter/flutter#83530. It puts files that are used to build new Flutter apps into paths expected by the new mechanism (which supports more than one type of app template).

[CrossFile] Fix class name in readme (flutter#383)

It says you have to use a class CrossFile which doesn't even exist in the package. It must be XFile.

[flutter_template_images] Fix maskable icons (flutter#363)

[flutter_lints]: analyzer_options.yaml -> analysis_options.yaml (flut…

…ter#368)

[pointer_interceptor] Shadow dom tweaks (flutter#364)

Update animations README (flutter#362)

Update README.md for flutter_lints (flutter#358)