Fix for Issue 7172.

Fix for Issue 7172.

Committed: https://code.google.com/p/dart/source/detail?r=15795

[sra1, 2012-12-06 03:13:04]

The original test was optimized to a field access and didn't use an interceptor
at all.

The generated code for B.foo is interesting - the getInterceptor calls for the
getter and setter are CSE-ed.
It is still much larger that the Dart source.

 foo$1: function(receiver) {
  var t1, t2;
  this.length = $.add(this.length, 1);
  t1 = $.getInterceptor$JSStringJSArray(receiver);
  t2 = t1.get$length(receiver);
  t1.set$length(receiver, $.add(t2, 1));
  return t2;
},

[ngeoffray, 2012-12-06 06:31:36]

LGTM!

[kasperl, 2012-12-06 07:50:10]

STV!