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

runtime/bin/builtin.dart

Index: runtime/bin/builtin.dart
diff --git a/runtime/bin/builtin.dart b/runtime/bin/builtin.dart
index 91702ab412621dfa77515676f8fc0b17cfd065dd..28ec419ce979168cf2ae0b132af586b8bd30fde1 100644
--- a/runtime/bin/builtin.dart
+++ b/runtime/bin/builtin.dart
@@ -310,3 +310,23 @@ String _filePathFromHttpUri(Uri uri) {
   _logResolution('# Path: $uri -> $uri');
   return uri.toString();
 }
+
+
+class _PlatformHook {
+  int get numberOfProcessors => Platform.numberOfProcessors;
+  String get pathSeparator => Platform.pathSeparator;
+  String get operatingSystem => Platform.operatingSystem;
+  String get localHostname => Platform.localHostname;
+  String get version => Platform.version;
+  Map<String, String> get environment => Platform.environment;
+  String get script => Platform.script;
+  String get executable => Platform.executable;
+  List<String>  get executableArguments => Platform.executableArguments;
+  String get packageRoot => Platform.packageRoot;
+}
+
+
+_getPlatform() => new _PlatformHook();
+
+
+