* Dart2JS build step looks for compiled platform binaries.

* Use new locations of platform binaries.

* Added --local-web-sdk command line flag.

* Need to use the matching frontend server when doing ddc stuff.

* Update packages/flutter_tools/lib/src/test/web_test_compiler.dart
Co-authored-by: Mouad Debbar <mouad.debbar@gmail.com>

* Update packages/flutter_tools/lib/src/runner/flutter_command_runner.dart
Co-authored-by: Mouad Debbar <mouad.debbar@gmail.com>

* Formatting issues.

* Need to use URI format for platform dill.

* Fix resident runner tests.

* Fix analysis issue.

* Fix and add unit tests.

* Add some useful comments.

* Refine doc comments for flags.
Co-authored-by: Mouad Debbar <mouad.debbar@gmail.com>