Make XMLHttpRequest.response available.

Make XMLHttpRequest.response available.

Not all types are available through responseText, responseXML etc.
In particular, arraybuffers need the .response getter.

The getter was missing from the Dart interfaces because it is declared
in the IDL as returning type 'Object' which is not declared anywhere.
Solved by parsing 'object' and 'Object' identically.

BUG=
TEST=

Committed: https://code.google.com/p/dart/source/detail?r=3778

[sra1, 2012-02-01 07:44:32]

tbr

[vsm, 2012-02-01 15:30:03]

LGTM!