MethodHandles: Add tests for MethodHandles.reflectAs

Also fix a corner case: reflectAs should throw a ClassCastException
if we're reflecting to the wrong type, regardless of the type of
LHS of the expression of which it's the RHS.

Test: vogar --mode=host MethodHandlesTest
Change-Id: I9d86e4e20fd9cac0eaa978cb10e000c5d2564d22