Move interop js code out of dart.js

Move interop js code out of dart.js

BUG=7973
Note: this is a breaking change and would be advertised as such.
Anyone using js-interop would need to explicitly include:

<script type="text/javascript" "pkg/browser/lib/interop.js"></script>

along with dart.js in their HTML.  Those who don't need js-interop can
save on this code.

Alexandre: I'm imagining interop.js would be the landing spot for
dart:js bootstrap code as well.

R=alexandre.ardhuin@gmail.com, dgrove@google.com

Committed: https://code.google.com/p/dart/source/detail?r=23855

[alexandre.ardhuin, 2013-06-06 20:07:35]

lgtm

[dgrove, 2013-06-06 23:16:33]

lgtm

[vsm, 2013-06-11 15:33:23]

Committed patchset #1 manually as r23855 (presubmit successful).