Move IterableMixinWorkaround to internal library.

Move IterableMixinWorkaround to internal library.

From here we can remove it in stages after M4 without risking breaking
the VM, dart2js or the HTML library right now.

Committed: https://code.google.com/p/dart/source/detail?r=21443

[floitsch, 2013-04-09 14:31:28]

LGTM.

Added Pete. For Pete's information: we are planning to replace the
IterableMixinWorkaround calls with mixins, but this way this is hidden behind
the scenes and we can do it a little bit later.

[blois, 2013-04-09 16:15:09]

lgtm

[kevmoo-old, 2013-04-09 19:33:22]

DBQ: What is IterableMixinWorkaround doing that's it's still needed in the dom
classes vs the new ListMixin?

[Lasse Reichstein Nielsen, 2013-04-11 15:15:28]

We just haven't gotten around to removing all the uses in the HTML library yet.
This requires someone with understanding of the code generation framework as
well.

[Lasse Reichstein Nielsen, 2013-04-15 12:35:16]

Committed patchset #2 manually as r21443 (presubmit successful).