Reland: Make File.create() fail when a directory exists at the same path

tests/standalone/io/file_create_test.dart

+// Copyright (c) 2016, 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 file creation.
+
+import 'dart:async';
+import 'dart:io';
+
+import "package:expect/expect.dart";
+import "package:path/path.dart";
+
+testCreate() async {
+  Directory tmp = await Directory.systemTemp.createTemp('file_test_create');
+  Expect.isTrue(await tmp.exists());
+  String filePath = "${tmp.path}/foo";
+  File file = new File(filePath);
+  File createdFile = await file.create();
+  Expect.equals(file, createdFile);
+  Expect.isTrue(await createdFile.exists());
+  await tmp.delete(recursive: true);
+}
+
+testBadCreate() async {
+  Directory tmp = await Directory.systemTemp.createTemp('file_test_create');
+  Expect.isTrue(await tmp.exists());
+  Directory tmp2 = await tmp.createTemp('file_test_create');
+  Expect.isTrue(await tmp2.exists());
+  String badFilePath = tmp2.path;
+  File badFile = new File(badFilePath);
+  try {
+    await badFile.create();
+    Expect.fail('Should be unreachable');
+  } catch(e) {
+    Expect.isTrue(e is FileSystemException);
+    Expect.isNotNull(e.osError);
+  }
+  await tmp.delete(recursive: true);
+}
+
+main() async {
+  await testCreate();
+  await testBadCreate();
+}