Add optional arguments to our indexOf/lastIndexOf methods.

runtime/lib/growable_array.dart

Index: runtime/lib/growable_array.dart
===================================================================
--- runtime/lib/growable_array.dart	(revision 942)
+++ runtime/lib/growable_array.dart	(working copy)
@@ -75,9 +75,6 @@
   // The length of this growable array. It is always less than the
   // length of the backing array.
   int _length;
-  // Constant used by indexOf and lastIndexOf when the element given
-  // is not in the array.
-  static final int ABSENT = -1;
 
   GrowableObjectArray()
       : _length = 0, backingArray = new ObjectArray<T>(4) {}
@@ -172,12 +169,13 @@
     return backingArray[_length - 1];
   }
 
-  int indexOf(T element, int startIndex) {
-    return Arrays.indexOf(backingArray, element, startIndex, _length);
+  int indexOf(T element, [int start = 0]) {
+    return Arrays.indexOf(backingArray, element, start, _length);
   }
 
-  int lastIndexOf(T element, int startIndex) {
-    return Arrays.lastIndexOf(backingArray, element, startIndex);
+  int lastIndexOf(T element, [int start = null]) {
+    if (start === null) start = length - 1;
+    return Arrays.lastIndexOf(backingArray, element, start);
   }
 
   /**