[clang-format] JS: don't treat is: as a type matcher

Summary:
Clang-format is treating all occurences of `is` in js as type matchers. In some
cases this is wrong, as it might be a dict key.

Reviewers: mprobst

Reviewed By: mprobst

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D54753

llvm-svn: 347307