[dart2js] invariance optimization for field setters under `-O2`

Calls to the implicit setters for fields are replaced with field assignments (HFieldSet). This is inhibited when the setter needs to check its argument, which happens when the field type is covariant and the checks not 'trusted', i.e. at optimizations level `-O2` and lower (aka 'spec' mode).

This optimization detects when the check is unnecessary due to invariance. If the receiver of a setter instance call to the the implicit setter has `this` as a receiver and the setter does not have an explicit `covariant` declaration, then the check will always pass, so the setter call can be replaced with HFieldSet.

This optimization works well for Iterators for generic collections at the assignment to the `_current` field, with some iteration-heavy benchmarks improved by 20-50% at `-O2`.

Change-Id: I4f3ffd08b45ba28de721535afc3c7a6d1e486a3d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/405448
Reviewed-by: Nate Biggs <natebiggs@google.com>
Commit-Queue: Stephen Adams <sra@google.com>