Change FileStat static methods to return NOT_FOUND instead for throwing

tests/standalone/io/file_stat_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.
 //
 // Dart test program for testing dart:io FileSystemEntity.Stat().
 
 import 'dart:async';
 import 'dart:io';
 
 import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 import "package:path/path.dart";
 
 void testStat() {
   Directory directory = Directory.systemTemp.createTempSync('dart_file_stat');
   File file = new File(join(directory.path, "file"));
-  Expect.throws(file.statSync);
-  Expect.throws(() => FileStat.statSync(file.path));
+  FileStat fileStat = FileStat.statSync(file.path);
+  FileStat fileStatDirect = file.statSync();
+  Expect.equals(FileSystemEntityType.NOT_FOUND, fileStat.type);
+  Expect.equals(FileSystemEntityType.NOT_FOUND, fileStatDirect.type);
   file.writeAsStringSync("Dart IO library test of FileStat");
   new Timer(const Duration(seconds: 2), () {
     file.readAsStringSync();
     directory.listSync();
     FileStat fileStat = FileStat.statSync(file.path);
     FileStat fileStatDirect = file.statSync();
     Expect.equals(FileSystemEntityType.FILE, fileStat.type);
     Expect.equals(32, fileStat.size);
     Expect.equals(FileSystemEntityType.FILE, fileStatDirect.type);
     Expect.equals(32, fileStatDirect.size);
@@ skipping 16 matching lines @@
     directory.deleteSync(recursive: true);
   });
 }
 
 
 Future testStatAsync() {
   return Directory.systemTemp.createTemp('dart_file_stat')
   .then((directory) {
     File file = new File(join(directory.path, "file"));
     return FileStat.stat(file.path)
-    .then((_) => Expect.fail("FileStat.stat should throw an exception."))
-    .catchError((e) => null)
+    .then((fileStat) => Expect.equals(FileSystemEntityType.NOT_FOUND,
+                                      fileStat.type))
     .then((_) => file.stat())
-    .then((_) => Expect.fail("File.stat should throw an exception."))
-    .catchError((e) => null)
+    .then((fileStat) => Expect.equals(FileSystemEntityType.NOT_FOUND,
+                                      fileStat.type))
     .then((_) => file.writeAsString("Dart IO library test of FileStat"))
     .then((_) => new Future.delayed(const Duration(seconds: 2)))
     .then((_) => file.readAsString())
     .then((_) => directory.list().last)
     .then((_) => FileStat.stat(file.path))
     .then((FileStat fileStat) {
       Expect.equals(FileSystemEntityType.FILE, fileStat.type);
       Expect.equals(32, fileStat.size);
       if (Platform.operatingSystem != 'windows') {
         Expect.isTrue(fileStat.modified.compareTo(fileStat.accessed) < 0);
@@ skipping 47 matching lines @@
     });
   });
 }
 
 
 void main() {
   asyncStart();
   testStat();
   testStatAsync().then((_) => asyncEnd());
 }