Implement type inference for await expressions in front_end.

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

Index: pkg/front_end/testcases/inference/future_then_upwards_from_block.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/future_then_upwards_from_block.dart.strong.expect b/pkg/front_end/testcases/inference/future_then_upwards_from_block.dart.strong.expect
new file mode 100644
index 0000000000000000000000000000000000000000..4c45b4243c3db3d091963520cff1ff1a4f69fde8
--- /dev/null
+++ b/pkg/front_end/testcases/inference/future_then_upwards_from_block.dart.strong.expect
@@ -0,0 +1,15 @@
+library test;
+import self as self;
+import "dart:async" as asy;
+import "dart:core" as core;
+
+static method test() → dynamic {
+  asy::Future<core::int> base;
+  asy::Future<core::bool> f = base.{asy::Future::then}<core::bool>((core::int x) → core::bool {
+    return x.{core::num::==}(0);
+  });
+  asy::Future<core::bool> g = base.{asy::Future::then}<core::bool>((core::int x) → core::bool => x.{core::num::==}(0));
+  asy::Future<core::bool> b = f;
+  b = g;
+}
+static method main() → dynamic {}