v0.16.0

Likely to cause new Flow errors:

• Some module exports that didn't require annotations before may now require annotations

New Features:

• Let/const support! Finally! (Huge props to @samwgoldman...again :] )
• Support for mixins on declare class library definitions

Notable Bug Fixes:

• Improvements to types inferred from switch cases that fall through to a default cause
• Further improvements on symlink support

Misc:

• Significant performance improvements (both for initial start-up time and running re-calculation time)
• Improved --traces output