Fix factories in Frog to correspond to the new syntax.

frog/leg/scanner/token.dart

 // Copyright (c) 2011, 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.
 
 final int EOF_TOKEN = 0;
 
 final int KEYWORD_TOKEN = $k;
 final int IDENTIFIER_TOKEN = $a;
 final int DOUBLE_TOKEN = $d;
 final int INT_TOKEN = $i;
@@ skipping 70 matching lines @@
 
   StringToken(int kind, String value, int charOffset)
     : this.fromSource(kind, new SourceString(value), charOffset);
 
   StringToken.fromSource(int kind, SourceString this.value, int charOffset)
     : super(kind, charOffset);
 
   String toString() => value.toString();
 }
 
-interface SourceString extends Hashable factory StringWrapper {
+interface SourceString extends Hashable default StringWrapper {
   const SourceString(String string);
 
   void printOn(StringBuffer sb);
 
   String get stringValue();
 }
 
 class StringWrapper implements SourceString {
   final String stringValue;
 
@@ skipping 10 matching lines @@
   }
 
   String toString() => stringValue;
 }
 
 class BeginGroupToken extends StringToken {
   Token endGroup;
   BeginGroupToken(int kind, String value, int charOffset)
     : super(kind, value, charOffset);
 }