v0.80.0

Likely to cause new Flow errors:

• We've changed how Flow lint severity is calculated. We used to look at every location mentioned by a lint and we'd use the lowest severity, where Off < Warn < Error. Now we just use the severity at the lint's first location. Flow suppression comments (e.g. // $FlowFixMe) can still suppress a lint at any mentioned location. Depending on your setup, this change may expose Flow lints which were turned off by accident. For example, a "Sketchy Null Check" lint in a file with that lint set to error would have been turned off if it also mentions a file with that lint off. Now that lint will show up as an error.
• Some libdef changes may cause a few errors. For example, Headers.get and URLSearchParams.get are now annotated to return null | string instead of string and File.lastModifiedDate: any was replaced with File.lastModified: number.

Notable bug fixes:

• Fixed a crash that could happen when a persistent connection disappears.

Misc:

• Many improvements to the libdefs and docs. Thanks for the PRs, everyone!
• Small perf improvement by making BoundT (internal representation of type parameter bounds) smaller.

Parser:

• Small change to the OCaml AST for the extends expression in a class declaration. Moved a couple of properties to their own node.