Fixes various Flutter framework test failures when bytecode is enabled.

Bytecode generation will otherwise omit assert code, causing --enable-asserts to have no effect in the VM.

Move tools tests into a general.shard directory in preparation to changing how we shard tools tests (#36108)

* Make it possible to override the FLUTTER_TEST env variable without unsetting it.

* Switch to using platform instead of Platform.

* Document the bindings, and introduce tests that initialize multiple WidgetsBindings with different environments.

* Add tests for the flutter platform test.

* Add license headers

* Fix lints

* Remove trailing whitespace

* Respond to Jonahs comments

* Respond to Ians comments

* Mock out the HttpServer in flutter_platform_test

* Mock out the HttpServer in flutter_platform_test

* Explain why we mock out the HttpServer in flutter_platform_test