dart2js: implement correct setter semantics

[lrhn]

The specification gives setters a separate name from getters, and name lookup of one does not interact with the other.

We currently assume that a getter (e.g., a final field) on a class introduces a "property", and that writing to the name fails due to a missing corresponding setter.

Instead writing to the name should skip the getter declaration completely and look further out for a setter for the name.

example:

set x(v) { print("OK"); };
class A {
  static final x = 42;
  static foo() { x = 10; }
}
main() => A.foo();

This program should print "OK" according to the specification. It currently reports that x is not assignable because the 'x' in the assignment is matched with the 'x' declaration in the class, and not the 'x setter' in the top scope.

[kasperl]

cc @larsbak.
Changed the title to: "dart2js: implement correct setter semantics".

[kasperl]

Removed this from the M1 milestone.
Added this to the M2 milestone.

[kasperl]

Removed this from the M2 milestone.

[kasperl]

Added this to the M2 milestone.

[peter-ahe-google]

Added Duplicate label.
Marked as being merged into #4870.