Implement type inference for await expressions in front_end.

pkg/front_end/testcases/inference/future_then_4.dart

Index: pkg/front_end/testcases/inference/future_then_4.dart
diff --git a/pkg/front_end/testcases/inference/future_then_4.dart b/pkg/front_end/testcases/inference/future_then_4.dart
index f2b1eba5c2f3ad0baba0b554759179f7d3153847..11033bdab106a327b5435146bf2bb9c7c667e949 100644
--- a/pkg/front_end/testcases/inference/future_then_4.dart
+++ b/pkg/front_end/testcases/inference/future_then_4.dart
@@ -14,7 +14,7 @@ class MyFuture<T> implements Future<T> {
   MyFuture<S> then<S>(FutureOr<S> f(T x), {Function onError}) => null;
 }
 
-void main() {
+void test() {
   MyFuture f;
   MyFuture<int> t1 = f. /*@typeArgs=int*/ /*@target=MyFuture::then*/ then(
       /*@returnType=Future<int>*/ (/*@type=dynamic*/ _) async =>
@@ -44,3 +44,5 @@ void main() {
     return new MyFuture<int>.value(3);
   });
 }
+
+main() {}