Distinguish between malformed and malbounded types (fix issues 12552 and 12554).

tests/language/f_bounded_quantification5_test.dart

+// Copyright (c) 2013, 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.
+
+// Test for F-Bounded Quantification.
+
+import "package:expect/expect.dart";
+
+class A<T extends B<T>> {
+}
+
+class B<T extends A<T>> {
+}
+
+isCheckedMode() {
+  try {
+    var i = 1;
+    String s = i;
+    return false;
+  } catch (e) {
+    return true;
+  }
+}
+
+main() {
+  bool got_type_error = false;
+  try {
+    Expect.equals("A<B<int>>", new A<B<int>>().runtimeType.toString());
+  } on TypeError catch (error) {
+    got_type_error = true;
+  }
+  // Type error expected in checked mode only.
+  Expect.isTrue(got_type_error == isCheckedMode());
+}
+