Use Pattern.matchAsPrefix to let String.indexOf/lastIndexOf/startsWith accept Pattern.

sdk/lib/core/string.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.
 
 part of dart.core;
 
 /**
  * The String class represents sequences of characters. Strings are
  * immutable. A string is represented by a sequence of Unicode UTF-16
  * code units accessible through the [codeUnitAt] or the
@@ skipping 87 matching lines @@
    * combining accent character "◌́".
    */
   bool operator ==(var other);
 
   /**
    * Returns whether this string ends with [other].
    */
   bool endsWith(String other);
 
   /**
-   * Returns whether this string starts with [other].
+   * Returns whether this string starts with a match of [pattern].
    */
-  bool startsWith(String other);
+  bool startsWith(Pattern pattern);
 
   /**
-   * Returns the first location of [other] in this string starting at
-   * [start] (inclusive).
-   * Returns -1 if [other] could not be found.
+   * Returns the first position of a match of [pattern] in this string,
+   * starting at [start] (inclusive).
+   *
+   * Returns -1 if a match could not be found.
+   *
+   * It is an error if start is negative or greater than [length].
    */
-  int indexOf(String other, [int start]);
+  int indexOf(Pattern pattern, [int start]);
 
   /**
-   * Returns the last location of [other] in this string, searching
+   * Returns the last position of a match [pattern] in this string, searching
    * backward starting at [start] (inclusive).
+   *
    * Returns -1 if [other] could not be found.
+   *
+   * It is an error if start is negative or greater than [length].
    */
-  int lastIndexOf(String other, [int start]);
+  int lastIndexOf(Pattern pattern, [int start]);
 
   /**
    * Returns whether this string is empty.
    */
   bool get isEmpty;
 
   /**
    * Returns whether this string is not empty.
    */
   bool get isNotEmpty;
@@ skipping 331 matching lines @@
         _position = position - 1;
         _currentCodePoint = _combineSurrogatePair(prevCodeUnit, codeUnit);
         return true;
       }
     }
     _position = position;
     _currentCodePoint = codeUnit;
     return true;
   }
 }