Use a Traverser in JSASTTest instead of a Transformer.
Using a Transformer in scalajsp is overkill.

It only needs to transform the top-level node, so a simple
function is enough.
Refactor Transformer with a single method transform().

Instead of overriding transformStat() and transformExpr(),
subclasses should now override transform(). This method takes
a boolean parameter telling whether it's used in statement
position.

Additionally, transformations of ClassDefs and their defs is
moved in a separate subclass ClassTransformer. This is actually
not used at all in our current codebase, but other people could
come up with use cases.
Merge pull request #1203 from sjrd/refactorings

Refactorings