Refactor EventEmitter and Async Facade to match ES7 Observable semantics, properly use RxJS typedefs, make EventEmitter inherit from RxJS Subject. Closes #4149.

BREAKING CHANGE:

• consumers of EventEmitter no longer need to call .toRx()
• EventEmitter is now generic and requires a type - e.g. EventEmitter<string>
• EventEmitter and Observable now use the .subscribe(generatorOrNext, error, complete) method instead of .observer(generator)
• ObservableWrapper uses callNext/callError/callComplete instead of callNext/callThrow/callReturn

Notes:

• This uses the RxJS main import path (though not the "kitchen sink"), and as such adds a number of methods to the public API (see public_api_spec). Note that Subject and EventEmitter both inherit from Observable and so it's less drastic than it looks in terms of new operators ;-) This is the current best workaround / compromise between having accurate type information and a usable EventEmitter / Observable interface. Reference discussions in Determine which Observable operators and extensions to include in angular core #4390, and over in Rx Move core operators onto Observable directly ReactiveX/rxjs#468 for a potential middle-term solution. This does add bytes to bundles, though with the incoming changes to bundling this could mostly be mitigated. In my opinion, this gives a better developer experience and we can watch over the beta period how and which operators should define the core.
• Currently this fails on CI due to the dgeni generation of types not liking Rx's external .d.ts files - see Remove deprecated dgeni dgeneration of d.ts files from gulp and CI #4967 but passes otherwise, and I'll rebase once that clears.
• It adds a psuedo Subject to the dart async facade that matches the EventEmitter - this could potentially be resolved by changes in ts2dart @mprobst ?
• tldr: this fixes a number of annoying issues re: developer experience (types and operators with EventEmitter, removes .toRx(), etc) and I think is a reasonable step to land before beta. I have a number of minor fixes (Http typedefs, etc) to follow to further improve said experience.