Change interfaces to abstract classes in dart2js compiler.

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 {
+abstract class Scanner {
   Token tokenize();
 }
 
 /**
  * Common base class for a Dart scanner.
  */
 class AbstractScanner<T extends SourceString> implements Scanner {
   abstract int advance();
   abstract int nextByte();
 
@@ skipping 837 matching lines @@
       next = advance();
     }
     return error(const SourceString("unterminated string literal"));
   }
 
   int error(SourceString message) {
     appendByteStringToken(BAD_INPUT_INFO, message);
     return advance(); // Ensure progress.
   }
 }