Make ImportDeclaration and ExportDeclaration semicolons spec-compliant

`ImportDeclaration`s end with a semicolon and `ExportDeclaration`s with specifiers end with a semicolon.

Reference: [Ecma-262 Edition 6 Rev 28 Section - 15.2.2 and 15.2.3](http://wiki.ecmascript.org/lib/exe/fetch.php?id=harmony%3Aspecification_drafts&cache=cache&media=harmony:working_draft_ecma-262_edition_6_10-14-14-nomarkup.pdf).