Allow patterns as argument of RestElement

[microbouji]

Q	A
Fixed Issues?	Fixes #8396, fixes #5673
Patch: Bug Fix?	Yes
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

Patterns as arguments to RestElements were apparently removed back in 9a97d92 when, as mentioned there, they were not part of es6. They're part of the standard now, so this PR adds them back in.

function foo(...[a]) {}

will now get converted to:

function foo() {
  for (var _len = arguments.length, _ref = new Array(_len), _key = 0; _key < _len; _key++) {
    _ref[_key] = arguments[_key];
  }

  var [a] = _ref;
}

letting the destructuring plugin handle it from there ([a] = _ref => a = _ref[0]).

Note that this does not make any of the optimizations that are otherwise made when the argument is an identifier - e.g. even though a is not referenced inside the function it will still be made available. If any optimizations are possible I think they can be added in other PRs?

I'll try to make a backport for v6 as well. EDIT: Don't think it's possible to change babylon 6.x anymore.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/8744/

[nicolo-ribaudo]

A possible loose mode optimization (it a dfferent PR) can be this one:

function foo(a, b, ...[c, d]) {}
-->
function foo(a, b, c, d) {}

[nicolo-ribaudo]

Thank you!