Downward inference of parameter types

[vsmenon]

In the closure in following code, we infer the return type (as int), but not the parameter types (left as dynamic):

typedef int Foo(int x, int y);

void bar() {
  Foo f = (x, y) => x+y;
}

That is sound, but suboptimal - x+y is handled via a dynamic invoke in generated code.

[leafpetersen]

Fixed by https://codereview.chromium.org/1462133005/ .