Adding annotation for partial browser support and example implementation for MutationObservers.

Adding annotation for partial browser support and example implementation for MutationObservers.


BUG=

Committed: https://code.google.com/p/dart/source/detail?r=16449

[sra1, 2012-12-21 02:49:21]

lgtm

[Emily Fortuna, 2012-12-21 18:57:38]

lgtm. Any reason why Experimental annotation can take arguments?

[blois, 2012-12-21 19:00:58]

On 2012/12/21 18:57:38, Emily Fortuna wrote:
> lgtm. Any reason why Experimental annotation can take arguments?

Could have used:
  const experimental = const Experimental();
then:
  @experimental
  class MutationObserver

But it's odd to have two syntaxes for annotations. The preferences voiced for
@deprecated and @override was to move both to @Override() so they would have
consistent syntax.