Implement type inference for await expressions in front_end.

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

Index: pkg/front_end/testcases/inference/future_union_downwards_generic_method_with_generic_return.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/future_union_downwards_generic_method_with_generic_return.dart.strong.expect b/pkg/front_end/testcases/inference/future_union_downwards_generic_method_with_generic_return.dart.strong.expect
new file mode 100644
index 0000000000000000000000000000000000000000..0c001f5016ba931be1ebdcc26bece374fcc253f0
--- /dev/null
+++ b/pkg/front_end/testcases/inference/future_union_downwards_generic_method_with_generic_return.dart.strong.expect
@@ -0,0 +1,36 @@
+library test;
+import self as self;
+import "dart:core" as core;
+import "dart:async" as asy;
+
+static method id<T extends core::Object>(self::id::T x) → self::id::T
+  return x;
+static method test() → dynamic /* originally async */ {
+  final asy::Completer<asy::FutureOr<dynamic>> :completer = asy::Completer::sync<asy::FutureOr<dynamic>>();
+  asy::FutureOr<dynamic> :return_value;
+  dynamic :async_op_then;
+  dynamic :async_op_error;
+  dynamic :await_jump_var = 0;
+  dynamic :await_ctx_var;
+  dynamic :saved_try_context_var0;
+  function :async_op([dynamic :result, dynamic :exception, dynamic :stack_trace]) → dynamic yielding 
+    try {
+      #L1:
+      {
+        asy::Future<core::String> f;
+        asy::_awaitHelper(self::id<asy::FutureOr<core::String>>(f), :async_op_then, :async_op_error, :async_op);
+        [yield] null;
+        core::String s = :result;
+      }
+      :completer.complete(:return_value);
+      return;
+    }
+    on dynamic catch(dynamic :exception, dynamic :stack_trace) {
+      :completer.completeError(:exception, :stack_trace);
+    }
+  :async_op_then = asy::_asyncThenWrapperHelper(:async_op);
+  :async_op_error = asy::_asyncErrorWrapperHelper(:async_op);
+  asy::Future::microtask<dynamic>(:async_op);
+  return :completer.future;
+}
+static method main() → dynamic {}