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

tests/lib/platform/isolate_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:platform" as platform;
+
+import "dart:isolate";
+import "package:unittest/unittest.dart";
+
+main() {
+  var sendPort = spawnFunction(f);
+  expect(sendPort.call("platform.executable"), completion(platform.executable));
+  if (platform.script != null) {
+    expect(sendPort.call("platform.script").then((s) => Uri.parse(s).path),
+           completion(endsWith('tests/lib/platform/isolate_test.dart')));
+  }
+  expect(sendPort.call("platform.packageRoot"),
+         completion(platform.packageRoot));
+  expect(sendPort.call("platform.executableArguments"),
+         completion(platform.executableArguments));
+}
+
+void f() {
+  int count = 0;
+  port.receive((msg, reply) {
+    if (msg == "platform.executable") {
+      reply.send(platform.executable);
+    }
+    if (msg == "platform.script") {
+      reply.send(platform.script);
+    }
+    if (msg == "platform.packageRoot") {
+      reply.send(platform.packageRoot);
+    }
+    if (msg == "platform.executableArguments") {
+      reply.send(platform.executableArguments);
+    }
+    count++;
+    if (count == 4) {
+      port.close();
+    }
+  });
+}