Clone all Traceur ASTs before mutating them

This avoids having to case-by-case squash bugs like the one that surfaced in 17c6e614dec8f2a7e3ba6c6be56b9246ff6bdb37, by ensuring that code never shares a parsed AST despite the PlaceholderParser's caching. See discussion in google/traceur-compiler#1407 that led to this solution.
Use parseStatement for import statements too

Not sure exactly when this became possible; see google/traceur-compiler#1271.