Evaluate JsObjectProxy instead of recursive conversion to map in `dartValue`.

[jakemac53]

Today given a plain JsObject whose constructor is context['Object'], we convert it to a map and recursively call convertToDart. This could potentially be pretty expensive, and also it loses track of the original JsObject, so you can't modify it. Instead we could create a JsObjectProxy, which is basically this:

class JsObjectProxy extends Object implements JsProxyInterface, JsObject {
  final JsObject jsProxy;

  JsObjectProxy(this.jsProxy);

  operator [](String key) => convertToDart(jsProxy[key]);

  operator []=(String key, value) => jsProxy[key] = convertToJs(value);
}

This would make it lazy, which is good, but repeated access of the same keys would come at a higher cost.

We could do the same for JsArray as well.

[jakemac53]

This would need to happen during a release candidate release, before official 1.0 as it would be a breaking change.

[jakemac53]

fyi @sigmundch

[jakemac53]

We could potentially implement the full Map class as well, but its not clear how the keys of the object should be iterated in order to do that (there are a few different ways of iterating keys of JS objects, none of which are guaranteed to do the "right" thing in all situations).