fixes#27625, Object constraints were not tracked in inference
fixes#27933, pin return type from downwards inference
We now prefer to pick the bound (lower or upper) that had some information on, and it also improves inference error messages somewhat (still a ways to go).
The way this works is we now have a type representing an unknown type: ?. We use ? when performing downward inference steps, instead of `dynamic`. This allows more accurate tracking of type constraints.
For example: given:
var x = await Future.wait([a, b]);
Future.wait<T>'s argument type is Iterable<Future<T>>. Since we didn't know T, we previously pushed down Iterable<Future<dynamic>>. The dynamic caused loss of information. Now we push down Iterable<Future<?>>, allowing us to infer the right type there.
R=leafp@google.com, vsm@google.com
Review-Url: https://codereview.chromium.org/2456803004 .
Vijay Menon