Proof-of-concept matchPrefix on Pattern.

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 403 matching lines @@
         break;
       } else if (position == endIndex) {
         ++startIndex;  // empty match, advance and restart
       } else {
         startIndex = endIndex;
       }
     }
     return result;
   }
 
+  Match matchAsPrefix(String string, [int start = 0]) {
+    if (start < 0 || start > string.length) {
+      throw new RangeError.range(start, 0, string.length);
+    }
+    if (start + this.length > string.length) return null;
+    for (int i = 0; i < this.length; i++) {
+      if (string.codeUnitAt(start + i) != this.codeUnitAt(i)) {
+        return null;
+      }
+    }
+    return new _StringMatch(start, string, this);
+  }
+
   List<String> split(Pattern pattern) {
     if ((pattern is String) && pattern.isEmpty) {
       List<String> result = new List<String>(length);
       for (int i = 0; i < length; i++) {
         result[i] = this[i];
       }
       return result;
     }
     int length = this.length;
     Iterator iterator = pattern.allMatches(this).iterator;
@@ skipping 167 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);
 }