Fixed T6675

[makenowjust]

I fixed T6675. This PR fixed the parser of directives like "use strict" because T6675 problem was caused by the bug existed there. For example, previous implementation cannot parse following code:

"use strict"
.split(" ").map(s => console.log(s))

Of course this code is correct. The factor of this problem is that it is tokens, not statements, that previous implementation uses for parsing directives. In other words, previous implementation regards "use strict" token as the directive as soon as reading this token, even if this token is one of the parts of the expression.

See ECMA-262 6th §14.1.1. I re-implement the parser of directives according to it.

[codecov-io]

Current coverage is 89.12%

Merging #3107 into master will decrease coverage by -0.05% as of 88b38cd

@@            master   #3107   diff @@
======================================
  Files          214     214       
  Stmts        15468   15461     -7
  Branches         0       0       
  Methods          0       0       
======================================
- Hit          13793   13779    -14
  Partial          0       0       
- Missed        1675    1682     +7

Review entire Coverage Diff as of 88b38cd

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[sebmck]

Thank you @makenowjust! Sorry for the delay in merging. I've been taking a break from OSS and Babel.