Minor fixes to remove some warnings.

lib/compiler/implementation/scanner/parser_bench.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.
 
 /**
  * A benchmark for the Dart parser.
  */
 class ParserBench extends BaseParserBench {
   int charCount = 0;
   double score = 0.0;
 
   Token scanFileNamed(String filename) {
     Token token;
     getBytes(filename, (bytes) {
-      AbstractScanner scanner = makeScanner(bytes);
+      StringScanner scanner = makeScanner(bytes);
       try {
         token = scanner.tokenize();
         printTokens(token);
         charCount += scanner.charOffset;
       } catch (MalformedInputException e) {
         print("${filename}: ${e}");
       }
     });
     return token;
   }
@@ skipping 79 matching lines @@
   }
 
   void beginClass(Token token) {
     classCount++;
   }
 
   void beginFunctionTypeAlias(Token token) {
     aliasCount++;
   }
 }