Add the ability to invoke private methods from native functions

[sgjesse]

For native function written in C (using the native keyword on the VM) it is not possible to invoke private methods. When a native method is invoked it seems logical that it is operating inside the library from which it is invoked and it should be possible to invoke private functions in that same library.

[turnidge]

Set owner to @turnidge.
Added Accepted label.

[turnidge]

Using Dart_Invoke it should now be possible to invoke private methods. (Note that Dart_InvokeState and Dart_InvokeDynamic are still broken, but are in the process of being removed.) Please reopen the bug if you run into a method that you can't invoke.

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Added Fixed label.