Add optional arguments to our indexOf/lastIndexOf methods.

runtime/lib/string.dart

Index: runtime/lib/string.dart
===================================================================
--- runtime/lib/string.dart	(revision 942)
+++ runtime/lib/string.dart	(working copy)
@@ -103,7 +103,7 @@
     return this.substringMatches(0, other);
   }
 
-  int indexOf(String other, int startIndex) {
+  int indexOf(String other, [int startIndex = 0]) {
     if (other.isEmpty()) {
       return startIndex < this.length ? startIndex : this.length;
     }
@@ -119,7 +119,8 @@
     return -1;
   }
 
-  int lastIndexOf(String other, int fromIndex) {
+  int lastIndexOf(String other, [int fromIndex = null]) {
+    if (fromIndex == null) fromIndex = length - 1;
     if (other.isEmpty()) {
       return Math.min(this.length, fromIndex);
     }