Fix build after analyzer_experimental API change.

pkg/polymer/lib/src/build/code_extractor.dart

 // Copyright (c) 2013, 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.
 
 /** Transfomer that extracts inlined script code into separate assets. */
 library polymer.src.build.code_extractor;
 
 import 'dart:async';
 
+import 'package:analyzer_experimental/src/generated/java_core.dart' show CharSequence;
 import 'package:analyzer_experimental/src/generated/ast.dart';
 import 'package:analyzer_experimental/src/generated/error.dart';
 import 'package:analyzer_experimental/src/generated/parser.dart';
 import 'package:analyzer_experimental/src/generated/scanner.dart';
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as path;
 
 import 'common.dart';
 
 /**
@@ skipping 45 matching lines @@
       }
       transform.addOutput(
           htmlChanged ? new Asset.fromString(id, document.outerHtml) : input);
     });
   }
 }
 
 /** Parse [code] and determine whether it has a library directive. */
 bool _hasLibraryDirective(String code) {
   var errorListener = new _ErrorCollector();
-  var token = new StringScanner(null, code, errorListener).tokenize();
+  var reader = new CharSequenceReader(new CharSequence(code));
+  var scanner = new Scanner(null, reader, errorListener);
+  var token = scanner.tokenize();
   var unit = new Parser(null, errorListener).parseCompilationUnit(token);
   return unit.directives.any((d) => d is LibraryDirective);
 }
 
 class _ErrorCollector extends AnalysisErrorListener {
   final errors = <AnalysisError>[];
   onError(error) => errors.add(error);
 }