Promise used to be typed as any, giving incorrect results. This change
fixes places that were incorrectly typed and re-exports the actual
Promise type from es6-promise.
It also fixes a series of compilation errors discovered/triggered by
this change.
The semantics between ES6 `Promise#all` and Dart's `Future#wait` are
different for values that are not `Promise`/`Future`s. In ES6,
non-`Promise` values are immediately completed to their current value.
In Dart, non-`Future` values cause an error.
Updated Dart's `PromiseWrapper#all` implementation to conform to the ES6
spec.