Fix: JsStackEmulatorTest failed under jvm 8.

When running in Java 8 classes are inserted in JProgram in a different
order causing fragile tests like this one to fail.

Change-Id: Ibaee8508fcbff7c4b7a7290fa2d8abb9c5766c6e
Drop effect on DataTransfer object.

This patch updates the data transfer object to implements the
drop effect.

For the dragenter and dragover events, the dropEffect will be
initialized based on what action the user is requesting. How this
is determined is platform specific, but typically the user can press
modifier keys to adjust which action is desired. Within an event
handler for the dragenter and dragover events, the dropEffect should
be modified if the action the user is requesting is not the one
that is desired.

For dragstart, drag, and dragleave events, the dropEffect is
initialized to "none". Any value assigned to the dropEffect will be
set, but the value isn't used for anything.

For the drop and dragend events, the dropEffect will be initialized
to the action that was desired, which will be the value that the
dropEffect had after the last dragenter or dragover event.

Change-Id: I09e75c35357d081c40f2827dd7b2e995b08f1a93