Fixing dart2js compilation warning.

Fixing dart2js compilation warning.

Was seeing the warnings:
build/ReleaseIA32/dart-sdk/lib/html/dart2js/html_dart2js.dart:15831:21: Warning: no property named _this in class Iterable
      if (identical(newNodes._this, this)) {
                    ^^^^^^^^^^^^^^
build/ReleaseIA32/dart-sdk/lib/html/dart2js/html_dart2js.dart:15838:27: Warning: no operator [] in class Iterable
        this.insertBefore(newNodes[0], refChild);

BUG=
R=efortuna@google.com

Committed: https://code.google.com/p/dart/source/detail?r=23428

[Emily Fortuna, 2013-05-30 17:41:15]

lgtm

[blois, 2013-05-30 17:47:12]

Committed patchset #1 manually as r23428 (presubmit successful).