Analyzer: control flow collections phase 1

[stereotype441]

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

Note that this should be behind an experimental flag.

See the language tracking issue: dart-lang/language#165
And the feature specification: https://github.com/dart-lang/language/blob/master/accepted/future-releases/control-flow-collections/feature-specification.md
And the implementation plan: https://github.com/dart-lang/language/blob/master/accepted/future-releases/control-flow-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)