Add fromUri constructor to File, Directory and Link

tests/standalone/io/directory_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('directory_uri').then((temp) {
+    String dirname = temp.path + '/from_uri';
+    Uri dirUri = new Uri.file(dirname);
+    Directory dir = new Directory.fromUri(dirUri);
+    Expect.isTrue(dirUri.isAbsolute);
+    Expect.isTrue(dirUri.path.startsWith('/'));
+    dir.createSync();
+    Expect.isTrue(new Directory.fromUri(dirUri).existsSync());
+    Expect.isTrue(
+        new Directory.fromUri(Uri.base.resolveUri(dirUri)).existsSync());
+    Directory.current = temp.path;
+    Expect.isTrue(new Directory.fromUri(Uri.parse('from_uri')).existsSync());
+    Expect.isTrue(
+        new Directory.fromUri(Uri.base.resolve('from_uri')).existsSync());
+    dir.deleteSync();
+    temp.deleteSync(recursive: true);
+    asyncEnd();
+  });
+}
+
+void testFromUriUnsupported() {
+  Expect.throws(
+      () => new Directory.fromUri(
+          Uri.parse('http://localhost:8080/index.html')),
+      (e) => e is UnsupportedError);
+  Expect.throws(
+      () => new Directory.fromUri(Uri.parse('ftp://localhost/tmp/xxx')),
+      (e) => e is UnsupportedError);
+  Expect.throws(() => new Directory.fromUri(Uri.parse('name#fragment')),
+                (e) => e is UnsupportedError);
+}
+
+void main() {
+  testFromUri();
+  testFromUriUnsupported();
+}