Handle implicit future unions in downwards inference

[leafpetersen]

Various context in Dart essentially treat Future<T> as an implicit union type Future<T> | T. For example, await allows either:

void test() async {
  List<int> x = await [4]; 
  List<int> y = await new Future.value([4]);
}

Downwards inference currently only considers the Future<int> possibility here, and hence will infer type parameters for the Future creation and the list literal in the second await, but not for the list literal in the first await. It would be useful to support both.

Other examples of this include the return value for the lambda argument to Future.then, and values returned from async functions:

Future<List<int>> test() async {
  return [4];  // Currently inferred
  return new Future.value([4]); // Currently not inferred
}

[jmesserly]

CC @trinarytree