Add extends clause to the type parameter for AbstractScanner.

lib/compiler/implementation/scanner/scanner.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.
 
 interface Scanner {
   Token tokenize();
 }
 
 /**
  * Common base class for a Dart scanner.
  */
-class AbstractScanner<T> implements Scanner {
+class AbstractScanner<T extends SourceString> implements Scanner {
   abstract int advance();
   abstract int nextByte();
   abstract int peek();
   abstract int select(int choice, PrecedenceInfo yes, PrecedenceInfo no);
   abstract void appendPrecedenceToken(PrecedenceInfo info);
   abstract void appendStringToken(PrecedenceInfo info, String value);
   abstract void appendByteStringToken(PrecedenceInfo info, T value);
   abstract void appendKeywordToken(Keyword keyword);
   abstract void appendWhiteSpace(int next);
   abstract void appendEofToken();
@@ skipping 735 matching lines @@
                                       charOffset);
   }
 }
 
 class MalformedInputException {
   final String message;
   final position;
   MalformedInputException(this.message, this.position);
   toString() => message;
 }