Add fields to platform library, implement them on runtime dart.

tests/standalone/io/uri_platform_test.dart

 // Copyright (c) 2012, 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";

[Anders Johnsen, 2013/10/24 08:30:08]

remove io import, and enable for dart2js.

[Bill Hesse, 2013/10/24 13:43:43]

Done.

+import "dart:platform" as platform;
 
 main() {
-  if (Platform.isWindows) {
+  if (platform.isWindows) {
     Expect.equals("a\\b", Uri.parse("a/b").toFilePath());
     Expect.equals("a\\b\\", Uri.parse("a/b/").toFilePath());
     Expect.equals("a b", Uri.parse("a%20b").toFilePath());
     Expect.equals("\\a b", Uri.parse("file:///a%20b").toFilePath());
     Expect.equals("\\a\\b", Uri.parse("file:///a/b").toFilePath());
     Expect.equals("C:\\", Uri.parse("file:///C:").toFilePath());
     Expect.equals("C:\\", Uri.parse("file:///C:/").toFilePath());
     Expect.equals("\\\\host\\a\\b", Uri.parse("file://host/a/b").toFilePath());
 
     Expect.equals("a\\b", new Uri.file("a/b").toFilePath());
@@ skipping 11 matching lines @@
     Expect.equals("/C:", Uri.parse("file:///C:").toFilePath());
     Expect.equals("/C:/", Uri.parse("file:///C:/").toFilePath());
     Expect.throws(() => Uri.parse("file://host/a/b").toFilePath(),
                   (e) => e is UnsupportedError);
 
     Expect.equals("a/b", new Uri.file("a/b").toFilePath());
     Expect.equals("a\\b", new Uri.file("a\\b").toFilePath());
   }
 
   Expect.equals(Uri.base,
-                new Uri.file(Directory.current.path + Platform.pathSeparator));
+                new Uri.file(Directory.current.path + platform.pathSeparator));
 }