Add assert for _FutureListener.handleValue receiver stack position

8f2c47f relies on the assumption that the receiver to _FutureListener.handleValue is at a fixed offset in the calling frame, but this assumption is not tested or guaranteed.

@mraleph suggested:

[add] checks when generating code for the handleValue itself.
e.g. [...] add a check in linearscan.cc that ParameterInstr for this is allocated into the right place on the stack when compiling handleValue body.
And similar check into LoadLocalInstr for unoptimized compilation.