Statically tracked shared immutable objects

This is a proposed solution to the problem of [communicating data between isolates](https://github.com/dart-lang/language/issues/124).

This proposal adds a static notion of immutability which guarantees that once initialized, any immutable object is the root of a fully immutable object graph, which can therefore be shared between isolates with no copying required.

This proposal also incidentally provides at least a partial solution for the following problems:
- [Building immutable collections](https://github.com/dart-lang/language/issues/117)
- [Unwanted mutation of lists in Flutter](https://github.com/dart-lang/sdk/issues/27755)

Initial draft proposal is [here](https://github.com/dart-lang/language/blob/master/working/0125-static-immutability/feature-specification.md)