trim HTML methods to reflect restrictions on Dart-DOM APIs (e.g. iframe.contentDocument...)

[sigmundch]

Some DOM functions are different than the standard Javascript DOM API, for instance, a parent window has restricted access to the DOM of a child frame via frame.contentDocument.

We should make sure to change the HTML library in a way that makes it easy to understand/discover these restrictions (e.g. the compiler should be able to issue warnings, runtime errors are understandable).

This is strongly related with this bug: http://code.google.com/p/dart/issues/detail?id=1173

[vsmenon]

Removed Area-UI label.
Added Area-DOM label.

[dgrove]

we've ripped out contentDocument. What else needs to be done here?

[vsmenon]

Added Duplicate label.
Marked as being merged into #2774.