Implement type inference for await expressions in front_end.

pkg/front_end/testcases/inference/future_or_subtyping.dart

Index: pkg/front_end/testcases/inference/future_or_subtyping.dart
diff --git a/pkg/front_end/testcases/inference/future_or_subtyping.dart b/pkg/front_end/testcases/inference/future_or_subtyping.dart
index e135a1cc98c127d949c2093f867e483b9b2a39f2..78752a1908d8ebf00ea06ebc21fe909bc3ff4e99 100644
--- a/pkg/front_end/testcases/inference/future_or_subtyping.dart
+++ b/pkg/front_end/testcases/inference/future_or_subtyping.dart
@@ -9,10 +9,12 @@ import 'dart:async';
 
 void add(int x) {}
 add2(int y) {}
-main() {
+test() {
   Future<int> f;
   var /*@type=Future<void>*/ a =
       f. /*@typeArgs=void*/ /*@target=Future::then*/ then(add);
   var /*@type=Future<dynamic>*/ b =
       f. /*@typeArgs=dynamic*/ /*@target=Future::then*/ then(add2);
 }
+
+main() {}