Add tests for throwing async return

Cf. #44395.

Change-Id: I753a6a4fae65c50267d8cf1575d2ce87e7ac345b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/175064
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
Commit-Queue: Erik Ernst <eernst@google.com>

tests/language/async/return_throw_test.dart

@@ -0,0 +1,23 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import "package:expect/expect.dart";
+
+Future<String> f() async {
+  throw 'f';
+}
+
+Future<String> g() async {
+  try {
+    // Should obtain the `Future<String>`, await it, then throw.
+    return f();
+  } catch (e) {
+    // Having caught the exception, we return a value.
+    return 'g';
+  }
+}
+
+void main() async {
+  Expect.equals('g', await g());
+}

tests/language_2/async/return_throw_test.dart

@@ -0,0 +1,23 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import "package:expect/expect.dart";
+
+Future<String> f() async {
+  throw 'f';
+}
+
+Future<String> g() async {
+  try {
+    // Should obtain the `Future<String>`, await it, then throw.
+    return f();
+  } catch (e) {
+    // Having caught the exception, we return a value.
+    return 'g';
+  }
+}
+
+void main() async {
+  Expect.equals('g', await g());
+}