Prepare for dart 3.0.0 alpha release

[annagrin]

In the dart 3.0.0 alpha release weak null safety mode will be disabled in build_web_compilers and flutter tools. See dart-lang/sdk#50700 for the proposed SDK changes.

Make sure we still can run weak null safety tests in dwds and webdev while various dependent tools will progressively remove null safety flags and artifacts.

• webdev, dwds tests: Do not pass --(no)-sound-null-safety flags to build_web_compilers

  • Prepare for dart 3.0 alpha breaking changes #1880

• dwds tests fixtures: Use special branch of build_web_compilers that supports weak null safety

  • Prepare for dart 3.0 alpha breaking changes #1880

• dwds tests (using build_web_compilers): generate missing SDK assets

  • Prepare for dart 3.0 alpha changes: generate assets #1887

After dart SDK 3.0.0-134.0.dev is released

• dwds tests: Handle moved and deleted SDK assets.

  • Prepare for dart 3.0 alpha changes: Handle SDK layout update #1907

• Prep for stable: Update SDK constraints to allow SDK <4.0.0

  • Allow dart SDK <4.0.0 #1913

• Release new versions of dwds and webdev

  • Prep DWDS for for 17.0.0 release #1924
  • Prepare Webdev for 3.0.0 release #1925

Cleanup

• dwds tests Move common test functionality into a common test package, use in dwds, webdev, etc.

  • Move common test functionality into package:test_common #1892
  • Move test-only code from the SdkLayout #1955
  • Create test_common package #1945

• webdev: Enable weak null safety tests

  • Enable weak null safety integration and e2e tests in webdev #1970
  • Re-enable weak webdev tests #1960

• dwds Make a copy of SDK directory to generate assets and run tests in

  • Run tests in a copy of the sdk directory #1985
  • Move project definitions into a separate class #1975
  • Create copying sdk configuration provider #1984
  • Pass SdkLayout to the frontend server #1986
  • Run webdev tests in a copy of the sdk directory #1987
  • Run dwds tests in a copy of the sdk directory #1989

• dwds Remove unused SDK configuration parts

  • Remove unused parts of SdkConfiguration #1910
  • Remove unused parts of SDK configuration #1911

Release

• g3 roll
• release dwds and webdev
• update dwds in flutter tools

[nshahan]

One more change to add to this list is that the path to find require.js packaged in the SDK will be changing.
dart-sdk/lib/dev_compiler/kernel/amd/require.js -> dart-sdk/lib/dev_compiler/amd/require.js

I plan on landing that in the same change that removes the .js files if possible.