Make String.startsWith take an optional start index.

runtime/lib/string_patch.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 patch class String {
   /* patch */ factory String.fromCharCodes(Iterable<int> charCodes) {
     return _StringBase.createFromCharCodes(charCodes);
   }
 }
 
@@ skipping 111 matching lines @@
         return false;
       }
     }
     return true;
   }
 
   bool endsWith(String other) {
     return _substringMatches(this.length - other.length, other);
   }
 
-  bool startsWith(Pattern pattern) {
+  bool startsWith(Pattern pattern, [int index = 0]) {
+    if (index < 0 || index > this.length) {
+      throw new RangeError.range(index, 0, this.length);
+    }
     if (pattern is String) {
-      return _substringMatches(0, pattern);
+      return _substringMatches(index, pattern);
     }
-    return pattern.matchAsPrefix(this, 0) != null;
+    return pattern.matchAsPrefix(this, index) != null;
   }
 
   int indexOf(Pattern pattern, [int start = 0]) {
     if (start < 0 || start > this.length) {
       throw new RangeError.range(start, 0, this.length);
     }
     if (pattern is String) {
       String other = pattern;
       int maxIndex = this.length - other.length;
       // TODO: Use an efficient string search (e.g. BMH).
@@ skipping 483 matching lines @@
 class _CodeUnits extends Object with ListMixin<int>,
                                      UnmodifiableListMixin<int> {
   /** The string that this is the code units of. */
   String _string;
 
   _CodeUnits(this._string);
 
   int get length => _string.length;
   int operator[](int i) => _string.codeUnitAt(i);
 }