Implement super operator calls

tests/language/src/SuperOperatorTest.dart

+// Copyright (c) 2011, 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.
+// Dart test for testing super operator calls
+
+
+class A {
+  String val = "";
+  operator + (String s) {
+    val = val + s;
+    return this;
+  }
+}
+
+class B extends A {
+  operator + (String s) {
+    super + (s + s);
+    return this;
+  }
+}
+
+main () {
+  var a = new A();
+  a = a + "William";
+  Expect.equals("William", a.val);
+
+  a = new B();
+  a += "Tell";
+  Expect.equals("TellTell", a.val);
+}