IPC could behave incorrectly for some message types

[attilah]

While I reading through the source code I found this possible problem:

https://github.com/Microsoft/vscode/blob/master/src/vs/base/parts/ipc/common/ipc.ts#L27

And this function used in the onMessage method and can cause an immediate return for RequestEventListen``, RequestEventDisposemessages based on theMessageType``` enum at the top.

I found no use case in the code where it should fail, but IMHO the isResponse method implementation is not correct and can cause future hard to catch problems.

[joaomoreno]

Oh man that's BAD. Great eye!

[joaomoreno]

Verifier: please code review.