> A Use Strict Directive may not contain an EscapeSequence or
LineContinuation.
It is `Use Strict Directive` spec, but the `expression` of `Directive`
isn't original string value, it has error if using it to codegen, so
here using `directive` of `Directive` to codegen and not to escape it.
Here is crashed test cases.
``` js
'use str\
ict';
```
The babel will print the original string, I follow it and avoid using
`print_str` because it will escape string.
I also changed some code using the `expression` of `Directive` to check
`Use Strict Directive` .