Add fromUri constructor to File, Directory and Link

tests/standalone/io/link_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('dart_file').then((temp) {
+    File target = new File(temp.path + '/target');
+    target.createSync();
+
+    String linkname = temp.path + '/from_uri';
+    Uri linkUri = new Uri.file(linkname);
+    Link link = new Link.fromUri(linkUri);
+    Expect.isTrue(linkUri.isAbsolute);
+    Expect.isTrue(linkUri.path.startsWith('/'));
+    link.createSync(target.path);
+    Expect.isTrue(new Link.fromUri(linkUri).existsSync());
+    Expect.isTrue(new Link.fromUri(Uri.base.resolveUri(linkUri)).existsSync());
+    Directory.current = temp.path;
+    Expect.isTrue(new Link.fromUri(Uri.parse('from_uri')).existsSync());
+    Expect.isTrue(new Link.fromUri(Uri.base.resolve('from_uri')).existsSync());
+    link.deleteSync();
+    target.deleteSync();
+    temp.deleteSync();
+    asyncEnd();
+  });
+}
+
+void testFromUriUnsupported() {
+  Expect.throws(
+      () => new Link.fromUri(Uri.parse('http://localhost:8080/index.html')),
+      (e) => e is UnsupportedOperation);
+  Expect.throws(() => new Link.fromUri(Uri.parse('ftp://localhost/tmp/xxx')),
+                (e) => e is UnsupportedOperation);
+  Expect.throws(() => new Link.fromUri(Uri.parse('name#fragment')),
+                (e) => e is UnsupportedOperation);
+}
+
+void main() {
+  testFromUri();
+  testFromUriUnsupported();
+}