Fix warnings in dart2js.

dart/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.
 
 abstract class Scanner {
   Token tokenize();
 }
 
 /**
  * Common base class for a Dart scanner.
  */
-class AbstractScanner<T extends SourceString> implements Scanner {
+abstract class AbstractScanner<T extends SourceString> implements Scanner {
   abstract int advance();
   abstract int nextByte();
 
   /**
    * Returns the current character or byte depending on the underlying input
    * kind. For example, [StringScanner] operates on [String] and thus returns
    * characters (Unicode codepoints represented as int) whereas
    * [ByteArrayScanner] operates on byte arrays and thus returns bytes.
    */
   abstract int peek();
@@ skipping 840 matching lines @@
       next = advance();
     }
     return error(const SourceString("unterminated string literal"));
   }
 
   int error(SourceString message) {
     appendByteStringToken(BAD_INPUT_INFO, message);
     return advance(); // Ensure progress.
   }
 }