[CP] Handle Object members of dart:html types in dart2js runtime

[srujzs]

commit(s) to merge: d172051

Please note that the only relevant non-test and non-status file changes are pkg/compiler/lib/src/js_emitter/native_emitter.dart and sdk/lib/_internal/js_runtime/lib/js_helper.dart, which amount to ~20 lines of code.

merge instructions: Cleanly cherry-picks to 455fe9d (the current beta version of 2.16.0-134.1.beta)

What is the issue: dart:html classes' type hierarchies were changed, but not fully accounted for in the dart2js runtime. This means we'd get incorrect values for some of the Object default methods e.g. hashCode.

What is the fix: The first change in the CL is to update the tags so dart2js can correctly register dart:html types. The second change is an additional if-clause to emit the right toString values.

Why cherrypick: The major component of this is hashCode. Maps that contain many instances of these types will hash them all in the same bucket, leading to performance regressions. Beyond this, and less importantly, toString and runtimeType result in significantly less useful values i.e. treating dart:html types as a generic JS object type.

Risk: Little.

The first change shouldn't make behavior worse than it already is, since it will only affect the dart:html types today. The second change only affects toString and error Strings when reporting a failed instance check.

Link to original issue(s): #48187

#47942 is where it first popped up.

/cc @mit-mit @whesse @athomas @vsmenon @devoncarew @sigmundch

[sigmundch]

I'm in favor of cherry-picking. This was an unintended regression and the cherry pick will bring 2.16 in line with the behavior and performance of 2.15.

Looking through the changes, the only actual code changes are down to 7 lines, the rest are comments and tests.

[devoncarew]

Thanks for the cherry pick request! We're working through the approval now (I believe mostly related to potentially validating this change via a google3 roll).

[devoncarew]

Updating this issue - this is now approved.

[srujzs]

Thanks Devon!

[whesse]

Cherry-picked into 2.16.0-134.5.beta.