The web platforms used to invent a spurious stack trace when reading
`Error.stackTrace` before the object was thrown.
They now return `null` instead, if there is no underlying JS error object,
matching specified behavior.
Fixed bugs in async error throwing in dart2wasm:
* `throw` in an async function did not set the stack trace on an error.
Now calls `Error._throw` instead of just a direct Wasm "throw".
* `async*` functions did not capture the stack trace of a throw
that ended the function body, which means it called
`StreamController.addError` with only one argument.
That then resused the stack trace from an `Error` throw instead
of the correct stack trace.
Added tests.
Change-Id: I1d9fa8d9e18076a7fe28254b60b950866cd550a7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/354021
Reviewed-by: Stephen Adams <sra@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Reviewed-by: Ömer Ağacan <omersa@google.com>
Commit-Queue: Lasse Nielsen <lrn@google.com>
f0aeebd0e7