Fix new service tests on windows; make status file more precise.

runtime/observatory/tests/service/next_through_simple_async_test.dart

 // Copyright (c) 2017, 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 'test_helper.dart';
 import 'service_test_common.dart';
 import 'dart:io';
 
 const int LINE_A = 13;
 const String file = "next_through_simple_async_test.dart";
 
 // line A is at the "code() async" line
 code() async {
-  File f = new File(Platform.script.path);
+  File f = new File(Platform.script.toFilePath());
   DateTime modified = await f.lastModified();
   bool exists = await f.exists();
   print("Modified: $modified; exists: $exists");
   foo();
 }
 
 foo() {
   print("Hello from Foo!");
 }
 
 List<String> stops = [];
 List<String> expected = [
   "$file:${LINE_A+0}:5", // on '(' in code()'
   "$file:${LINE_A+1}:30", // on 'script'
-  "$file:${LINE_A+1}:37", // on 'path'
+  "$file:${LINE_A+1}:37", // on 'toFilePath'
   "$file:${LINE_A+1}:16", // on File
   "$file:${LINE_A+2}:31", // on 'lastModified'
   "$file:${LINE_A+2}:23", // on 'await'
   "$file:${LINE_A+3}:25", // on 'exists'
   "$file:${LINE_A+3}:17", // on 'await'
   "$file:${LINE_A+4}:47", // on ')', i.e. before ';'
   "$file:${LINE_A+4}:3", // on call to 'print'
   "$file:${LINE_A+5}:3", // on call to 'foo'
   "$file:${LINE_A+6}:1" // ending '}'
 ];
 
 var tests = [
   hasPausedAtStart,
   setBreakpointAtLine(LINE_A),
   runStepThroughProgramRecordingStops(stops),
   checkRecordedStops(stops, expected)
 ];
 
 main(args) {
   runIsolateTestsSynchronous(args, tests,
       testeeConcurrent: code, pause_on_start: true, pause_on_exit: true);
 }