dart2js backend support for spreads and control flow features [meta bug]

[sigmundch]

Both features are supported by the same work:

• migrating to use the CFE constants
  • Add js semantics to CFE constants (add js number semantics to kernel constants #36179)
  • Integrate CFE constants in the compiler (integrate kernel constants with inference and ssa #36178)
• implement block expressions (integrate block expressions in dart2js #36177)

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current status: updated April 2, 2019

• js-semantics CL reviewed and expected to land today [April 2, 2019]
• believe integration of constants is done, waiting for confirmation to close the bug.
• block expressions: done!