Avoid generating an overriding field in dart:dom

Avoid generating an overriding field in dart:dom

- Detect when the shadowed implementation is adequate and use that.
- If it is not adequate, generate getter / setter

BUG=
TEST=

Committed: https://code.google.com/p/dart/source/detail?r=4193

[sra1, 2012-02-12 18:01:18]

What I like about this change:
- avoids the need to define the meaning of a shadowed native field (which must
be different to non-native shadowed field)
- removes some code from dart:dom

What I don't like:
- 50 lines of Python to change 50 lines in generated code.

[Jacob, 2012-02-13 19:11:00]

lgtm