Add optional arguments to our indexOf/lastIndexOf methods.

runtime/lib/array.dart

Index: runtime/lib/array.dart
===================================================================
--- runtime/lib/array.dart	(revision 942)
+++ runtime/lib/array.dart	(working copy)
@@ -116,12 +116,13 @@
     DualPivotQuicksort.sort(this, compare);
   }
 
-  int indexOf(T element, int startIndex) {
-    return Arrays.indexOf(this, element, startIndex, this.length);
+  int indexOf(T element, [int start = 0]) {
+    return Arrays.indexOf(this, element, start, this.length);
   }
 
-  int lastIndexOf(T element, int startIndex) {
-    return Arrays.lastIndexOf(this, element, startIndex);
+  int lastIndexOf(T element, [int start = null]) {
+    if (start === null) start = length - 1;
+    return Arrays.lastIndexOf(this, element, start);
   }
 
   Iterator<T> iterator() {
@@ -243,12 +244,13 @@
     return "ImmutableArray";
   }
 
-  int indexOf(T element, int startIndex) {
-    return Arrays.indexOf(this, element, startIndex, this.length);
+  int indexOf(T element, [int start = 0]) {
+    return Arrays.indexOf(this, element, start, this.length);
   }
 
-  int lastIndexOf(T element, int startIndex) {
-    return Arrays.lastIndexOf(this, element, startIndex);
+  int lastIndexOf(T element, [int start = null]) {
+    if (start === null) start = length - 1;
+    return Arrays.lastIndexOf(this, element, start);
   }
 
   Iterator<T> iterator() {