Sky should expose services provided to/by embedder

These now appear on internals as takeServicesProvidedToEmbedder and
takeServicesProvidedByEmbedder. When you call these functions, you get back the
raw int that represents the handle. We'll need to wrap them up inside the Dart
VM with the appropriate types. We can create a nice wrapper for that in a
future CL.

R=hansmuller@google.com, hansmuller@chromium.org

Review URL: https://codereview.chromium.org/944733002

examples/dart-mojo.sky

@@ -74,7 +74,7 @@ main() {
   var messagePipe = new MojoMessagePipe();
   var wget = new WGet(messagePipe.endpoints[1]);
   wget.listen();
-  var shellProxy = new shell_mojom.ShellProxy.fromHandle(new MojoHandle(internals.passShellProxyHandle()));
+  var shellProxy = new shell_mojom.ShellProxy.fromHandle(new MojoHandle(internals.takeShellProxyHandle()));
   wget.initializeFromShellProxy(shellProxy, ["mojo:wget", "http://www.google.com"], "mojo:wget");
 }