Support rest params in package:js

[ochafik]

A @varargs or @rest annotation would be useful in two use-cases:

• To implement ES6 JS helpers with rest params in DDC (see Write all of the runtime in Dart. dart-archive/dev_compiler#310):

  mixin(base, @js.varargs mixins) {...}

  Would be compiled by DDC to:

  function mixin(base, ...mixins) {...}

• To call external (or internal) methods with rest params. The following call:

  foo(a, b, js.varargs(others)) // or js.rest(others)

  Would be compiled to ES6 code by DDC:

  foo(a, b, ...others)

  And to something like the following ES5 code by dart2js (some devil in the this binding details):

  foo.apply(null, [a, b].concat(others))

[mit-mit]

Clearing out 1.15 milestone as the last full push to dev has happened. If any changes are required before 1.15 is shipped, please file a merge request

[jacob314]

Closing. We expect to instead leverage possible future core dart language features for rest parameters.