Do not allow platform prefices to conflict with fixed members.

Do not allow platform prefices to conflict with fixed members.

Currently when generating prefices for platform libs we can generate
a name which equals to some fixed member.  Example, there is
ImageElement.x getter in dart:html which makes 'x' into a set
of forbidden member names.

Now imagine the class:

import 'dart:math' as math;

class Foo {
  get x => null;
  f() => math.cos(.5);
}

We retain 'x', but we can generate 'x' as a prefix for dart:math emitting:

import 'dart:math' as x;

class C {
  get x => null;
  f() => x.cos(.5);
}

Caboooooom!

R=smok@google.com
BUG=5674
Committed: https://code.google.com/p/dart/source/detail?r=13468

[Roman, 2012-10-10 11:26:44]

lgtm

Looks like writing a test should be easy? :)