Add uri getter on file system entity.

tests/standalone/io/file_system_uri_test.dart

+// Copyright (c) 2015, 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 "package:expect/expect.dart";
+import "dart:io";
+
+testFile(String input) {
+  if (Platform.isWindows) {
+    input = input.replaceAll('/', '\\');
+  }
+  var file = new File(input);
+  var uri = file.uri;
+  var file2 = new File.fromUri(uri);
+  Expect.equals(file.path, file2.path, input);
+}
+
+testDirectory(String input, [String output]) {
+  if (output == null) output = input;
+  if (Platform.isWindows) {
+    input = input.replaceAll('/', '\\');
+    output = output.replaceAll('/', '\\');
+  }
+  var dir = new Directory(input);
+  var uri = dir.uri;
+  var dir2 = new Directory.fromUri(uri);
+  Expect.equals(output, dir2.path, input);
+}
+
+void main() {
+  testFile("");
+  testFile("/");
+  testFile("foo/bar");
+  testFile("/foo/bar");
+  testFile("/foo/bar/");
+
+  testDirectory("", "/");
+  testDirectory("/");
+  testDirectory("foo/bar", "foo/bar/");
+  testDirectory("/foo/bar", "/foo/bar/");
+  testDirectory("/foo/bar/");
+}