DDC leaks parent window monkey-patches

[nex3]

If DDC-compiled code in an iframe calls window.parent, DDC injects a bunch of monkey-patches into the parent Window's prototype. If the iframe is later killed, these monkey-patches remain; what's worse, on Chrome they stop the frame's entire DOM tree from being garbage collected. This can create a memory leak when a DDC app is loaded in a child frame, even if that app only ever communicates with the parent window using something like window.postMessage() that doesn't involve sending Dart objects back and forth.