Add fromUri constructor to File, Directory and Link

tests/standalone/io/file_uri_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.
+
+import 'dart:io';
+
+import "package:async_helper/async_helper.dart";
+import "package:expect/expect.dart";
+
+void testFromUri() {
+  asyncStart();
+  Directory.systemTemp.createTemp('file_uri').then((temp) {
+    String filename = temp.path + '/from_uri';
+    Uri fileUri = new Uri.file(filename);
+    File file = new File.fromUri(fileUri);
+    Expect.isTrue(fileUri.isAbsolute);
+    Expect.isTrue(fileUri.path.startsWith('/'));
+    file.createSync();
+    Expect.isTrue(new File.fromUri(fileUri).existsSync());
+    Expect.isTrue(new File.fromUri(Uri.base.resolveUri(fileUri)).existsSync());
+    Directory.current = temp.path;
+    Expect.isTrue(new File.fromUri(Uri.parse('from_uri')).existsSync());
+    Expect.isTrue(new File.fromUri(Uri.base.resolve('from_uri')).existsSync());
+    file.deleteSync();
+    temp.deleteSync(recursive: true);
+    asyncEnd();
+  });
+}
+
+void testFromUriUnsupported() {
+  Expect.throws(
+      () => new File.fromUri(
+          Uri.parse('http://localhost:8080/index.html')),
+      (e) => e is UnsupportedError);
+  Expect.throws(() => new File.fromUri(Uri.parse('ftp://localhost/tmp/xxx')),
+                (e) => e is UnsupportedError);
+  Expect.throws(() => new File.fromUri(Uri.parse('name#fragment')),
+                (e) => e is UnsupportedError);
+}
+
+void main() {
+  testFromUri();
+  testFromUriUnsupported();
+}