Likely to cause new Flow errors:

• Fixed React.PureComponent's definition, so previously missed errors are now reported
• The definition of console in the build-in libdef has been filled out and is no longer any.

New Features:

• Forward references in type annotations: you can now reference a class in a type annotation before the class is declared
• T is now covariant in Class<T>. So if class B extends class A, then Class<B> is now a subtype of Class<A>
• Flow now lets you destructure objects with computed keys.
• flow check-contents --respect-pragma - check-contents checks whatever you give it, regardless of @flow or @noflow. This option changes the behavior to respect the pragma.
• flow check-contents will now report parsing errors too (thanks @nmote!)

Notable bug fixes:

• Fixed --trace behavior when we unify types
• Fixed situation where the client could spin-wait for the server
• Fixed non-termination triggered by certain calls to Function.prototype.{apply,call}
• Fixed issue where "Duplicate module provider" errors would stick around even after being fixed.
• (Windows) Fixed how the flow client tails the flow server's logs
• (Windows) Fixed the log rotating
• (Windows) Fixed issues where Flow would report Process Handles instead of Process IDs, leading to bad messages and a non-functional flow stop
• (Windows) Fixed build outside of a git or hg repository
• (Windows) Better error messages when you have paths that are too long

Misc:

• undefined is now just a global variable declared in the libdefs
• Various built-in libdef improvements
• Nifty PR from @nmn which teaches flow that nullableArr.filter(Boolean) is non-nullable