Add support for --compiler flag

[jakemac53]

Closes #1776

Overview

• Adds a new Compiler class, similar to the Runtime class, but it doesn't support custom ones (at least for now).
  • This is exported from package:test_api/backend.dart
• Adds required defaultCompiler and supportedCompilers fields to Runtime. This is breaking for custom runtimes.
  • Platforms will not be asked to load tests for compilers that the associated runtimes don't support.
• Add compiler field to SuitePlatform. This is how platforms get access to compiler configuration. It means there is no API breaking change for platforms, but they do need to be updated to support the configuration.
  • This is breaking, the compiler parameter is required when creating a SuitePlatform, although it is nullable, but defaults to the default compiler for the runtime. This is mostly for convenience and up for discussion.
• Adds support in all platforms to respect the compiler option.
  • The vm platform supports the source compiler which is roughly equivalent to --use-data-isolate-strategy. The --use-data-isolate-strategy flag is now just an alias for --compiler source and it it deprecated/hidden.
    • We would call this compiler none but we already have OS.none, so it would conflict in boolean selectors.
• Adds a CompilerSelection class similar to RuntimeSelection, used by SuiteConfig to track the selected compiler(s).
• Adds new compilerSelections field to SuiteConfig.
• Adds the compilers to the valid boolean selectors, this makes them usable in TestOn etc.

Still to do

• Consider merging the wasm support into the regular browser platform. Unclear if we want to do this now or wait. We could do it and then just call the compiler --experimental-dart2wasm?
• Write tests
• Update changelogs/pubpsecs
• Write docs
• Probably drop or deprecate --use-data-isolate-strategy

[jakemac53]

@natebosch I forgot to send it as a PR, but please take a look at 3748acb which updates the pubspecs and changelogs

[jakemac53]

Ok, this PR is ready for a final review now. As discussed offline we decided to leave the wasm support as is for now, given that chrome requires extra special flags to run in that mode, it makes sense to be a separate platform still.