Add optional arguments to our indexOf/lastIndexOf methods.

compiler/lib/implementation/string.js

 // 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.
 
 function native_StringImplementation__indexOperator(index) {
   return this[index];
 }
 
 function native_StringImplementation__charCodeAt(index) {
   return this.charCodeAt(index);
 }
 
 function native_StringImplementation_get$length() {
   return this.length;
 }
 
 function native_StringImplementation_EQ(other) {
   return typeof other == 'string' && this == other;
 }
 
-function native_StringImplementation_indexOf(other, startIndex) {
+function native_StringImplementation__nativeIndexOf(other, startIndex) {
   return this.indexOf(other, startIndex);
 }
 
-function native_StringImplementation_lastIndexOf(other, fromIndex) {
+function native_StringImplementation__nativeLastIndexOf(other, fromIndex) {
   if (other == "") {
     return Math.min(this.length, fromIndex);
   }
   return this.lastIndexOf(other, fromIndex);
 }
 
 function native_StringImplementation_concat(other) {
   return this.concat(other);
 }
 
@@ skipping 81 matching lines @@
     }
     array = tmp;
   }
   return String.fromCharCode.apply(this, array);
 }
 
 // Deprecated old name of new String.fromValues(..).
 function native_StringBase_createFromCharCodes(array) {
   return native_StringImplementation__newFromValues(array);
 }