MethodCallTargetNode: also simplify default method calls to virtual calls if possible.

--HG--
extra : rebase_source : 20a33554eb0d0d5daf941d701c3a9d08ed1cffbd
MethodCallTargetNode: document the single implementor optimization.

--HG--
extra : rebase_source : 698e1e575b0b07d27a0947f41efa42321510960c
MethodCallTargetNode: replace StampFactory.declared() with StampFactory.declaredNonNull().

--HG--
extra : rebase_source : 9379964075b33b88cec66e7bab8b4e4d3fb57d65