Implement type inference for await expressions in front_end.

pkg/front_end/testcases/inference/future_or_subtyping.dart.strong.expect

Index: pkg/front_end/testcases/inference/future_or_subtyping.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/future_or_subtyping.dart.strong.expect b/pkg/front_end/testcases/inference/future_or_subtyping.dart.strong.expect
new file mode 100644
index 0000000000000000000000000000000000000000..4f6fa47f36d9b483a3e9934f035d1cd641238388
--- /dev/null
+++ b/pkg/front_end/testcases/inference/future_or_subtyping.dart.strong.expect
@@ -0,0 +1,13 @@
+library test;
+import self as self;
+import "dart:core" as core;
+import "dart:async" as asy;
+
+static method add(core::int x) → void {}
+static method add2(core::int y) → dynamic {}
+static method test() → dynamic {
+  asy::Future<core::int> f;
+  asy::Future<void> a = f.{asy::Future::then}<void>(self::add);
+  asy::Future<dynamic> b = f.{asy::Future::then}<dynamic>(self::add2);
+}
+static method main() → dynamic {}