fields that implement extension members don't work

[jmesserly]

Split from #520

The following test fails, because defineExtensionMember does not find a property descriptor for MyList.length, so it does not create [dartx.length] and thus ListMixin.forEach will throw.

import 'dart:collection';
import 'package:expect/expect.dart';

class MyList extends ListBase {
  int length = 4;
  int operator[](int x) => 42;
  void operator[]=(int x, val) {}
}

main() {
  var x = new MyList();
  int z = 0;
  // runtime error in here, but the bug was in how MyList type was set up
  x.forEach((y) { z += y; });
  Expect.equals(z, 4 * 42);
}

[jmesserly]

@vsmenon @jacob314 -- thoughts on priority of this one?

[jmesserly]

(also I think Jacob's fix might address this?)

[jacob314]

I don't think my CL impacts this as I only change behavior for native classes with extensions.

[vsmenon]

Here's another example I'm seeing (with same MyList above):

main() {
  dynamic x = new MyList();
  print(x.isEmpty);
}

This prints undefined. Same result if we replace dynamic with var.

[jmesserly]

ah chatted about that example, it's probably the same issue. Any ListBase/Mixin that uses a length field will be broken (because that is what ListMixin does: implement all members in terms of length and [] []=). Either we need to know to emit a getter/setter pair for that at compile time, or the defineExtensionMembers helper needs to know that it should do so.

There's also a related issue that "length" shows up twice in the defineExtensionMembers list (probably once from the getter and once from the setter)

[vsmenon]

Found a workaround, lowering back to P2.

This may be a different bug - changing length to a getter did not fix.

import 'dart:collection';

class MyList extends ListBase with ListMixin {
  int get length => 4;
  void set length(int x) {}
  int operator[](int x) => 42;
  void operator[]=(int x, val) {}
}

main() {
  dynamic x = new MyList();
  print(x.isEmpty);
}

[vsmenon]

Ok, syncing up and now the getter version works. Sorry for the noise.