Constant propagation in LoadIndexed when inputs are array/string and index constants.

tests/language/load_indexed_constant_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 constant propgation of load-indexed operations
+// VMOptions=--optimization-counter-threshold=10
+
+import "package:expect/expect.dart";
+
+
+main() {
+  Expect.equals(101, stringIndexedLoad());
+  Expect.equals(102, arrayIndexedLoad());
+  for (int i = 0; i < 20; i++) {
+    stringIndexedLoad();
+    arrayIndexedLoad();
+  }
+  Expect.equals(101, stringIndexedLoad());
+  Expect.equals(102, arrayIndexedLoad());
+}
+
+stringIndexedLoad() => ("Hello").codeUnitAt(1);
+arrayIndexedLoad() => (const [101, 102, 103])[1];