* implicit-casts:false on flutter_tools

* use castStringKeyedMap

* address review comments

* address review comments

* fix issues after rebase

Removes multiple re-entrant calls of bundle and aot and replaces them with a single call to assemble. This restores full caching and will allow follow-up performance improvements when building multiple ABIs

* remove Source.behavior, fix bug in depfile invalidation

* more cleanup of assets

* Add skip

* address comments

* Update build_system.dart

This reverts commit 3597bae6.

Fixes various Flutter framework test failures when bytecode is enabled.

...because otherwise, processes that think they're manipulating your
filesystem will be doing crazy things the test is ignoring, leading to
(at best) failures and (at worst) flakes or disk corruption.

Bytecode generation will otherwise omit prologue code, causing --causal-async-stacks passed to the VM to behave inconsistently.

Reland "AOT support for Linux Desktop I: switch Linux builds to assemble (#41612)" (#42030)" (#42038)

This reverts commit 142a8630.

This reverts commit 3024053c.

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

* Stop using build_runner for dart2js

* fixes to yield when computing hashes and to imports

* add missing await

* Update filecache_test.dart

* Fix paths in filecache test

* use file uri for import

* add test cases and configurable override

* remove test dep

* fix filepaths for windows

* test no longer failing

* fix paths for test cases

* fix typo

* address comments

* make a constant

* make filehash async and use constant

* fix silly logic error