Add optional arguments to our indexOf/lastIndexOf methods.

corelib/src/string.dart

Index: corelib/src/string.dart
===================================================================
--- corelib/src/string.dart	(revision 942)
+++ corelib/src/string.dart	(working copy)
@@ -50,14 +50,14 @@
    * [startIndex] (inclusive).
    * Returns -1 if [other] could not be found.
    */
-  int indexOf(String other, int startIndex);
+  int indexOf(String other, [int startIndex]);
 
   /**
    * Returns the last location of [other] in this string, searching
    * backward starting at [fromIndex] (inclusive).
    * Returns -1 if [other] could not be found.
    */
-  int lastIndexOf(String other, int fromIndex);
+  int lastIndexOf(String other, [int fromIndex]);
 
   /**
    * Returns whether this string is empty.
@@ -92,7 +92,7 @@
    * Returns whether this string contains [other] starting
    * at [startIndex] (inclusive).
    */
-  bool contains(Pattern other, int startIndex);
+  bool contains(Pattern other, [int startIndex]);
 
   /**
    * Returns a new string where the first occurence of [from] in this string