split out GatherImports to its own pass (even though it will end up not being one eventually)
we only need the .prototype.constructor assignment if there is a superclass.  also, fix the assignment in the es6 code (it was assigning %super.prototype.constructor, not ${n.id}.prototype.constructor.
add more ES6 tests

several tests from kangax's table (unicode codepoint escapes, Symbol.hasInstance, Symbol.toStringTag), as well
as a test for shorthand methods in object literals.