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

tests/lib/platform/executable_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.
 
-// Test of parameterized types with invalid bounds.
+import "dart:platform" as platform;
 
-class A<K extends int> {
-}
+import "package:unittest/unittest.dart";
 
-class B<X, Y> {
-  foo(x) {
-    return x is A<X>;  /// 00: dynamic type error
+main() {
+  if (platform.operatingSystem != null) {
+    expect(platform.executable, contains('dart'));
+  } else {
+    expect(platform.executable, isNull);
   }
 }
 
-main() {
-  var b = new B<double, double>();
-  b.foo(new A());
-}