[dart2wasm] Potentially have equivalent of `@pragma('dart2js:late:trust')` / `@pragma('dart2js:as:trust')`

[mkustermann]

While comparing code size between dart2wasm and dart2js I've noticed that the ACX code contains usages of @pragma('dart2js:late:trust') / @pragma('dart2js:as:trust') in the code.

One pattern is to construct a unsafeCast method that can be used in performance critical code

@pragma('dart2js:tryInline') // Required for next pragma to be effective.
@pragma('dart2js:as:trust') // Omits `as T`.
T unsafeCast<T>(dynamic any) => any as T;

another is to remove checks for late fields for autogenerated code, it globally disables late checks for a library:

@pragma('dart2js:late:trust')

class X extends ComponentView {
  late final field;

  @override
  void build() {
    field = ...;
  }
}

While I'm not advocating for introducing these unsafe options in dart2wasm right now, having a way to enable them with a flag, would allow a more eye-to-eye comparison of dart2js and dart2wasm.

First we should make late smaller in code size (see #60432) then we could evaluate how much size we could safe by having support for similar annotations in dart2wasm.

It may be that this only really gives value in cases like ACX where we have large amounts of autogenerated code.

[mkustermann]

/cc @osa1 just fyi

[rakudrama]

I added @pragma('dart2js:late:trust') specifically for ACX generated code. Most of the benefit is not in checking before assignment, but in checking when reading. Generally, there are more reads than writes, and the compiler can't tell that build() must succeed before, say, detectChanges() can be called.

The other place that it is used is in the constructor of the protobuf base class GeneratedMessage.
It is important (1) to avoid checks doing this._fieldSet on every access to a protobuf field and, to a lesser extent, (2) to be able to GVN the reads of this field. The difficultly in doing this from first principles is that the partially initialized GeneratedMessage is passed to new FieldSet and the FieldSet constructor calls methods on the GeneratedMessage.