dart2js needs to implement metadata describing a native field of a native type

[rakudrama]

The html library implementation needs a way to indicate that a DOM class has a field with a Dart name that maps to a JavaScript property with a different name.

  @­JS_NAME('nextSibling') // JavaScript name
  final _NodeImpl nextNode; // Dart name

This is better than the currently available 'inline assembly' version:

  _NodeImpl get nextNode() => JS('_NodeImpl', '#.nextSibling', this);

The declarative metadata form is better because JS() is opaque, so dart2js can't tell that inlining the JS() body is possible on a nullable receiver. Inlining on nullable receivers opens up many more inlining possiblities but is only possible if the inlined code preserves the NullPointerException behaviour.

In addition mapping nextNode to nextSibling, the declarative form tells the compiler that nextSibling exists on native classes so should not be used as the JavaScript member name of any Dart members.

[rakudrama]

Marked this as being blocked by #4058.

[kasperl]

Added this to the Later milestone.

[kasperl]

Removed this from the Later milestone.

[kasperl]

Added this to the M2 milestone.

[peter-ahe-google]

Set owner to @rakudrama.

[peter-ahe-google]

Removed this from the M2 milestone.
Added this to the Later milestone.

[rakudrama]

dart:html now used @­JSName annotations on many fields and methods.

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Added Fixed label.