Try caching attribute map to try to improve performance.

Try caching attribute map to try to improve performance.

BUG=
R=blois@google.com

Committed: https://code.google.com/p/dart/source/detail?r=27844

[Emily Fortuna, 2013-09-24 19:23:33]

This is a test to see if we can improve performance for attribute[] so that
possibly we could make getAttribute private and remove it from the Dromoaeo
dom-attr.dart test. 

I'm committing this given the inconsistency I experienced while testing locally.


Regardless of the outcome of this test, the results will allow us to remove the
extra temporary dromaeo performance tests.

[blois, 2013-09-24 19:35:17]

This is an optimization for dromaeo, but results in overall memory increase for
the application- if something walks the DOM looking for a specific attribute on
an element, then every element will have this cached.

Is it worth the tradeoff?

[Emily Fortuna, 2013-09-24 19:39:02]

the point is to see how much of a performance gain this brings about, so that we
can have that very discussion. Like I said, this isn't a permanent change. This
is just a test.

[blois, 2013-09-24 19:44:12]

lgtm

[Emily Fortuna, 2013-09-24 19:52:13]

Committed patchset #1 manually as r27844 (presubmit successful).