clang-format: [JS] handle object types in extends positions.

Summary:
clang-format would previously drop the whitespace after `extends` in code such as:

    class Foo extends {} {}

Where the first set of curly braces is an inline object literal type.

Reviewers: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D36131

llvm-svn: 309695