Language: Implement "Spread Collections"

[munificent]

The "Spread Collections" proposal (#47) has been accepted and is being implemented (yay!). This is the parent issue to track all of the specific issues for implementing the feature across various tools and libraries.

Phase 0: Prerequisite

• [x] Experimental flag (TODO)
• [x] Language tests (TODO)

Phase 1: Foundation

• [x] Implementation in CFE/Kernel (No tracking bug, feature is implemented)
• [ ] Implementation in analyzer (https://github.com/dart-lang/sdk/issues/35569)

Phase 2: Tool implementation

• [x] Implementation in dart2js (https://github.com/dart-lang/sdk/issues/36313)
• [x] Implementation in dartfmt (https://github.com/dart-lang/dart_style/issues/778)
• [x] Implementation in dartdevc (https://github.com/dart-lang/sdk/issues/36006)
• [ ] Implementation in IntelliJ (TODO)
• [ ] Implementation in Grok (TODO)
• [ ] Implementation in analyzer (https://github.com/dart-lang/sdk/issues/35570)
• [x] Implementation in VM (https://github.com/dart-lang/sdk/issues/36218)
• [ ] Co19 tests (TODO)

Phase 3: Release

• [ ] Enable (TODO)
• [ ] Use in SDK and team-maintained packages (TODO)
• [ ] Documentation (TODO)

Phase 4: Clean-up

• [ ] Remove flag (TODO)

[munificent]

Note this open question: https://github.com/dart-lang/language/issues/167

[munificent]

A first batch of tests are out for review here: https://dart-review.googlesource.com/c/sdk/+/89085

[munificent]

dartfmt support is landed in the dart_style repo, but not published and rolled into the SDK yet. I'll do that once, I can upgrade it to analyzer 0.36.0.

[leafpetersen]

Is this complete? Can we close this?

[munificent]

Yes!