Interpreter: Fix issue where the interpreter was exiting too early
Fixes 222, CloneTreeTransformation fails on trees with empty lists.
transformList now returns new list if the input is empty, it's another kind of leaf.
Add test that cloned tree does not overlap original.
BUG=https://github.com/google/traceur-compiler/issues/222
Merge pull request #223 from johnjbarton/issue222

Fixes 222, CloneTreeTransformation fails on trees with empty lists.
Merge pull request #228 from arv/async-node-demo

Interpreter: Fix issue where the interpreter was exiting too early