Fix the stack_trace frame test on Windows as well.

pkg/stack_trace/test/trace_test.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.
 
 library trace_test;
 
+import 'dart:io';
 import 'dart:uri';
 
 import 'package:pathos/path.dart' as path;
 import 'package:stack_trace/stack_trace.dart';
 import 'package:unittest/unittest.dart';
 
 String getStackTraceString() {
   try {
     throw '';
   } catch (_, stackTrace) {
@@ skipping 66 matching lines @@
       var trace = nestedGetCurrentTrace(2);
       expect(trace.frames.first.member, equals('main.<fn>.<fn>'));
     });
 
     test('throws an ArgumentError for negative levels', () {
       expect(() => new Trace.current(-1), throwsArgumentError);
     });
   });
 
   test('.toString() nicely formats the stack trace', () {
-    var absolute = path.absolute(path.join('foo', 'bar.dart'));
+    var uri = _pathToFileUri(path.join('foo', 'bar.dart'));
     var trace = new Trace.parse('''
-#0      Foo._bar (file://$absolute:42:21)
+#0      Foo._bar ($uri:42:21)
 #1      zip.<anonymous closure>.zap (dart:async:0:2)
 #2      zip.<anonymous closure>.zap (http://pub.dartlang.org/thing.dart:1:100)
 ''');
 
     expect(trace.toString(), equals('''
 ${path.join('foo', 'bar.dart')} 42:21                        Foo._bar
 dart:async                                zip.<fn>.zap
 http://pub.dartlang.org/thing.dart 1:100  zip.<fn>.zap
 '''));
   });
@@ skipping 19 matching lines @@
 ''');
 
     expect(trace.terse.toString(), equals('''
 foo.dart 42:21  notCore
 dart:core       bottom
 bar.dart 10:20  alsoNotCore
 dart:async      bottom
 '''));
   });
 }
+
+String _pathToFileUri(String pathString) {
+  pathString = path.absolute(pathString);
+  if (Platform.operatingSystem != 'windows') return 'file://$pathString';
+  return 'file:///${pathString.replaceAll("\\", "/")}';
+}