JS backend: fix default arguments

generate special statements, like this:
 if (arg1 === void(0))
    arg1 = value1
JS backend: fix order call super constructor
JS backend: remove wrong usages program().getStringLiteral(). Use context().getNameForDescriptor()
JS backend: add super call support
JS backend: add simple test connected with frontend bug 'isStatement' for function body == 'hasBlockBody'
JS backend: fix override property
JS backend: fix init val in constructor