Add support for watch expressions.

[indiejames]

Right now setting a watch on a variable returns an exception. Apparently watch calls eval to get the value of an expression instead of allowing the debug adapter to set the watch.

[indiejames]

I added support for watch expressions to the eval call, but encountered a race condition. We want to use reval to evaluate the expression in a frame context after a breakpoint is hit, but the breakpoint event is reported and triggers the reval call before JDI seems to be ready for it. If I set a breakpoint in my debugger code and catch it then continue it, the call works. If I don't JDI complains that the call must be made at a breakpoint.

[indiejames]

Haven't figured this out yet, so I'm dropping this from milestone 1.