Fix build after analyzer_experimental API change.

pkg/observe/lib/transform.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.
 
 /**
  * Code transform for @observable. The core transformation is relatively
  * straightforward, and essentially like an editor refactoring.
  */
 library observe.transform;
 
 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:source_maps/refactor.dart';
 import 'package:source_maps/span.dart' show SourceFile;
 
 /**
  * A [Transformer] that replaces observables based on dirty-checking with an
@@ skipping 60 matching lines @@
             span: _getSpan(sourceFile, declaration));
       }
     }
   }
   return code;
 }
 
 /** Parse [code] using analyzer_experimental. */
 CompilationUnit _parseCompilationUnit(String code) {
   var errorListener = new _ErrorCollector();
-  var scanner = new StringScanner(null, code, errorListener);
+  var reader = new CharSequenceReader(new CharSequence(code));
+  var scanner = new Scanner(null, reader, errorListener);
   var token = scanner.tokenize();
   var parser = new Parser(null, errorListener);
   return parser.parseCompilationUnit(token);
 }
 
 class _ErrorCollector extends AnalysisErrorListener {
   final errors = <AnalysisError>[];
   onError(error) => errors.add(error);
 }
 
@@ skipping 300 matching lines @@
 
 Token _findFieldSeperator(Token token) {
   while (token != null) {
     if (token.type == TokenType.COMMA || token.type == TokenType.SEMICOLON) {
       break;
     }
     token = token.next;
   }
   return token;
 }