Fix for analyzer_cli/test/package_prefix_test on Windows.

pkg/analyzer_cli/test/package_prefix_test.dart

 import 'dart:io' show exitCode;
 
 import 'package:analyzer_cli/src/driver.dart' show Driver, outSink, errorSink;
 import 'package:analyzer_cli/src/options.dart' show ExitHandler, exitHandler;
+import 'package:path/path.dart';
 import 'package:unittest/unittest.dart';
 
 import 'utils.dart' show testDirectory;
 
 main() {
   group('--x-package-warnings-prefix', () {
     _Runner runner;
 
     setUp(() {
       runner = new _Runner.setUp();
     });
 
     tearDown(() {
       runner.tearDown();
       runner = null;
     });
 
     test('shows only the hint whose package matches the prefix', () {
       runner.run([
         "--packages",
-        "$testDirectory/data/package_prefix/packagelist",
+        join(testDirectory, 'data', 'package_prefix', 'packagelist'),
         "--x-package-warnings-prefix=f",
-        "$testDirectory/data/package_prefix/main.dart"
+        join(testDirectory, 'data', 'package_prefix', 'main.dart')
       ]);
-      print("ran it");
       expect(runner.stdout, contains('1 hint found'));
       expect(runner.stdout, contains('Unused import'));
-      expect(runner.stdout, contains('package_prefix/pkg/foo/foo.dart'));
+      expect(runner.stdout,
+          contains(join('package_prefix', 'pkg', 'foo', 'foo.dart')));
       expect(runner.stdout, isNot(contains('bar.dart')));
     });
   });
 }
 
 class _Runner {
   final _stdout = new StringBuffer();
   final _stderr = new StringBuffer();
 
   final StringSink _savedOutSink;
   final StringSink _savedErrorSink;
   final int _savedExitCode;
   final ExitHandler _savedExitHandler;
 
   _Runner.setUp()
       : _savedOutSink = outSink,
         _savedErrorSink = errorSink,
         _savedExitHandler = exitHandler,
         _savedExitCode = exitCode {
     outSink = _stdout;
     errorSink = _stderr;
     exitHandler = (_) {};
   }
 
+  String get stderr => _stderr.toString();
+
+  String get stdout => _stdout.toString();
   void run(List<String> args) {
     new Driver().start(args);
     if (stderr.isNotEmpty) {
       fail("Unexpected output to stderr:\n$stderr");
     }
   }
 
-  String get stdout => _stdout.toString();
-  String get stderr => _stderr.toString();
-
   void tearDown() {
     outSink = _savedOutSink;
     errorSink = _savedErrorSink;
     exitCode = _savedExitCode;
     exitHandler = _savedExitHandler;
   }
 }