Stop creating selectors in the SSA builder that were already created by the resolver.

tests/language/super_operator_index2_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 super indexing operations.
+
+class A {
+  var map = new Map();
+  operator[]=(a, b) { map[a] = b; }
+  operator[](a) => map[a];
+}
+
+class B extends A {
+  foo() {
+    super[4] = 42;
+    Expect.equals(42, super[4]);
+    super[4] += 5;
+    Expect.equals(47, super[4]);
+  }
+}
+
+main() {
+  new B().foo();
+}