Support checked mode for return type.

tests/language/return_type_test.dart

+// Copyright (c) 2012, 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.
+
+isCheckedMode() {
+  try {
+    var i = 1;
+    String s = i;
+    return false;
+  } catch (e) {
+    return true;
+  }
+}
+
+int returnString1() => 's';
+void returnNull() => null;
+void returnString2() => 's';
+
+main() {
+  if (isCheckedMode()) {
+    Expect.throws(returnString1, (e) => e is TypeError);
+    Expect.throws(returnString2, (e) => e is TypeError);
+    returnNull();
+  } else {
+    returnString1();
+    returnNull();
+    returnString2();
+  }
+}