Diagnose unterminated "grouping" characters.

dart/tests/language/src/UnbalancedBraceTest.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.
+
+// Test syntax errors related to unterminated braces.
+
+class A {
+  m() {
+  /* /// 01: compile-time error
+  }
+  // */
+
+/* /// 02: compile-time error
+}
+// */
+
+class B {
+}
+
+main() {
+  new A();
+  new B();
+}