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

• Experimental flag (TODO)
• Language tests (TODO)

Phase 1: Foundation

• Implementation in CFE/Kernel (No tracking bug, feature is implemented)
• Implementation in analyzer (Analyzer: spread collections phase 1 sdk#35569)

Phase 2: Tool implementation

• Implementation in dart2js (dart2js backend support for spreads and control flow features [meta bug] sdk#36313)
• Implementation in dartfmt (Implement spread in collection literals dart_style#778)
• Implementation in dartdevc (dartdevc: Implement "Spread Collections" sdk#36006)
• Implementation in IntelliJ (TODO)
• Implementation in Grok (TODO)
• Implementation in analyzer (Analyzer: spread collections phase 2 sdk#35570)
• Implementation in VM (VM: Implement "Spread Collections" sdk#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: #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.