Acorn release 4.0.0

A new version of Acorn is out, with partial ES8 support!

Breaking changes

The default ecmaVersion option value is now 7.

A number of internal method signatures changed, so plugins might need
to be updated.

Bug fixes

The parser now raises errors on duplicated export names.

arguments and eval can now be used in shorthand properties.

Duplicate parameter names in non-simple argument lists now always
produce an error.

New features

The ecmaVersion option now also accepts year-style version numbers
(2015, etc).

Support for async/await syntax when ecmaVersion is >= 8.

Support for trailing commas in call expressions when ecmaVersion is >= 8.