Add optional arguments to our indexOf/lastIndexOf methods.

client/observable/observable.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.
 
 #library('observable');
 
 #import('dart:coreimpl');
 
 #source('ChangeEvent.dart');
 #source('EventBatch.dart');
@@ skipping 192 matching lines @@
         return false;
       }
       return true;
     });
     if (found != null) {
       recordListRemove(index, found);
     }
     return found;
   }
 
-  int indexOf(T element, int startIndex) {
-    return _internal.indexOf(element, startIndex);
+  int indexOf(T element, [int start = 0]) {
+    return _internal.indexOf(element, start);
   }
 
-  int lastIndexOf(T element, int startIndex) {
-    return _internal.lastIndexOf(element, startIndex);
+  int lastIndexOf(T element, [int start = null]) {
+    if (start === null) start = length - 1;
+    return _internal.lastIndexOf(element, start);
   }
 
   bool removeFirstElement(T element) {
     // the removeAt above will record the event.
     return (removeAt(indexOf(element, 0)) != null);
   }
 
   int removeAllElements(T element) {
     int count = 0;
     for (int i = 0; i < length; i++) {
@@ skipping 59 matching lines @@
     //              equality check should be done?
     if (newValue !== _value) {
       final oldValue = _value;
       _value = newValue;
       recordPropertyUpdate("value", newValue, oldValue);
     }
   }
 
   T _value;
 }