Remove DartObject from the global JS namespace, remove interop.js

[justinfagnani]

DartObject pollutes the JavaScript namespace and creates a single proxy class for all Dart objects regardless of the isolate they originate from, potentially allowing Dart object to leak across isolates.

If we emit a DartObject function/prototype directly from dart2js it'll be accessible from dart:js without being in the global namespace.

[sethladd]

Do we still intend to do this?