[dart2wasm] Implement `sync*`

This is an implementation of `sync*` via two main mechanisms:

- The closure context infrastructure is used for preserving local
  state. All local variables in `sync*` functions are implicitly
  captured in the contexts even if they are not captured by a lambda.

- Suspension and resumption of the body is implemented via a state
  machine as a switch in a loop. This allows for an arbitrary control
  flow graph that can be resumed at any point. A subclass of the code
  generator generates control constructs containing any `yield` or
  `yield*` statements as jumps around this CFG while delegating the
  rest of the code generation to the normal member code generator.

This version does not support `switch` or `try` inside a `sync*`
function. Support for these statements will be added later.

Change-Id: Iec8236f64500d823f574aa628ddb0d22fe4ac2d0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/280166
Reviewed-by: Joshua Litt <joshualitt@google.com>
Commit-Queue: Aske Simon Christensen <askesc@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>