Analyzer: spread collections phase 1

[stereotype441]

Phase 1 covers foundational support, i.e. parsing the new syntax, type checking it, and evaluating ... in constant collections.

Note that this should be behind an experimental flag.

See the language tracking issue: dart-lang/language#164
And the feature specification: https://github.com/dart-lang/language/blob/master/accepted/future-releases/spread-collections/feature-specification.md
And the implementation plan: https://github.com/dart-lang/language/blob/master/accepted/future-releases/spread-collections/implementation-plan.md

Remaining tasks include:

• Record additional implicit downcasts to support DDC (still determining whether additional work is needed here)
• Warn if the feature is used in a project with too old an SDK constraint Analyzer should warn when using UI as code features with an SDK constraint that does not guarantee support #36046
• Fix language and co19 test failures
• Ensure we’re reporting errors for deferred library references in const references
• Summary support
• Follow up on failing tests tagged in the repo with Analyzer: spread collections phase 1 #35569
• Fix any remaining failures that appear when experimentally enabling the feature (see https://dart-review.googlesource.com/c/sdk/+/95660)

[stereotype441]

Note that the language team is currently writing tests for this feature: https://dart-review.googlesource.com/c/sdk/+/89085

[bwilkerson]

I have checked off the classes for which the necessary changes have already been committed. I have pending changes for several of the remaining classes, but didn't want to mark them as complete before they're committed.

Is SourceVisitor the class by that name in dart_style, or the class named _SourceVisitor in analyzer? If it's the former, then it probably shouldn't be on this list. If it's the latter then we should rename it and mark it complete (because it doesn't require any changes).

[stereotype441]

Is SourceVisitor the class by that name in dart_style, or the class named _SourceVisitor in analyzer? If it's the former, then it probably shouldn't be on this list. If it's the latter then we should rename it and mark it complete (because it doesn't require any changes).

It was the one in dart_style. I've removed it from the list.