Fix analyzer_experimental hints: remove unused imports.

pkg/analyzer_experimental/example/scanner_driver.dart

 #!/usr/bin/env 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.
 
-import 'package:analyzer_experimental/src/generated/java_core.dart';
 import 'package:analyzer_experimental/src/generated/scanner.dart';
 
 import 'dart:io';
 
 main() {
 
   print('working dir ${new File('.').fullPathSync()}');
 
   var args = new Options().arguments;
   if (args.length == 0) {
     print('Usage: scanner_driver [files_to_scan]');
     exit(0);
   }
 
   for (var arg in args) {
     _scan(new File(arg));
   }
 
 }
 
 _scan(File file) {
   var src = file.readAsStringSync();
   var scanner = new StringScanner(null, src, null);
   var token = scanner.tokenize();
   while (token.type != TokenType.EOF) {
     print(token);
     token = token.next;
   }
 }