Fix build after analyzer_experimental API change.

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

Index: pkg/polymer/lib/src/build/code_extractor.dart
diff --git a/pkg/polymer/lib/src/build/code_extractor.dart b/pkg/polymer/lib/src/build/code_extractor.dart
index 5e9135d71696dcc047c1d24ed7708ebedf05c7cf..1890b9816bb244dff191f061a2e1eef3b2bc4523 100644
--- a/pkg/polymer/lib/src/build/code_extractor.dart
+++ b/pkg/polymer/lib/src/build/code_extractor.dart
@@ -7,6 +7,7 @@ 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';
@@ -72,7 +73,9 @@ class InlineCodeExtractor extends Transformer with PolymerTransformer {
 /** 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);
 }