Make String.startsWith take an optional start index.

sdk/lib/core/string.dart

Index: sdk/lib/core/string.dart
diff --git a/sdk/lib/core/string.dart b/sdk/lib/core/string.dart
index 0d2503a8e2c89dacaabd1d681dc69b4121432871..3524139e7e44c0cd19798038fa83ccecad422e18 100644
--- a/sdk/lib/core/string.dart
+++ b/sdk/lib/core/string.dart
@@ -106,8 +106,13 @@ abstract class String implements Comparable<String>, Pattern {
 
   /**
    * Returns whether this string starts with a match of [pattern].
+   *
+   * If [index] is provided, instead check if the substring starting

[floitsch, 2013/06/17 16:07:15]

If [index] is provided, checks if `this` starts with [pattern] at position
[index].

Give example with Regexp that has "^".

+   * at that index starts with [pattern].
+   *
+   * It is an error if [index] is negative or greater than [length].
    */
-  bool startsWith(Pattern pattern);
+  bool startsWith(Pattern pattern, [int index = 0]);
 
   /**
    * Returns the first position of a match of [pattern] in this string,