[CS2] use _extends utility instead of Object.assign() for object spreads (#4675)

* _extends utility instead of Object.assign()

* eqJS test for _extends

* Test that a user-defined function named `_extends` doesn’t conflict with our utility function

* IE8 polyfill note in docs

Author: Julian Rosse

documentation/sections/es2015plus_output.md

@@ -11,3 +11,5 @@ npm install --global coffeescript@next
 npm install --save-dev coffeescript@next babel-cli babel-preset-env
 coffee --print *.coffee | babel --presets env > app.js
 ```
+
+Note that [babel-preset-env](https://babeljs.io/docs/plugins/preset-env/) doesn’t automatically supply polyfills for your code. CoffeeScript itself will output [`Array.indexOf`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/indexOf) if you use the `in` operator, or destructuring or spread/rest syntax; and [`Function.bind`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind) if you use a bound (`=>`) method in a class. Both are supported in Internet Explorer 9+ and all more recent browsers, but you will need to supply polyfills if you need to support Internet Explorer 8 or below and are using features that would cause these methods to be output, or in your own code are using similarly modern methods. One option is [`babel-polyfill`](https://babeljs.io/docs/usage/polyfill/), though there are many [other](https://hackernoon.com/polyfills-everything-you-ever-wanted-to-know-or-maybe-a-bit-less-7c8de164e423) [strategies](https://philipwalton.com/articles/loading-polyfills-only-when-needed/).

lib/coffeescript/nodes.js

@@ -1530,7 +1530,8 @@ exports.Obj = class Obj extends Base
         propSlices.push prop
     addSlice()
     slices.unshift new Obj unless slices[0] instanceof Obj
-    (new Call new Literal('Object.assign'), slices).compileToFragments o
+    _extends = new Value new Literal utility '_extends', o
+    (new Call _extends, slices).compileToFragments o
 
   compileCSXAttributes: (o) ->
     props = @properties
@@ -3728,6 +3729,19 @@ UTILITIES =
       }
     }
   "
+  _extends: -> "
+    Object.assign || function (target) {
+      for (var i = 1; i < arguments.length; i++) {
+        var source = arguments[i];
+        for (var key in source) {
+          if (Object.prototype.hasOwnProperty.call(source, key)) {
+            target[key] = source[key];
+          }
+        }
+      }
+      return target;
+    }
+  "
 
   # Shortcuts to speed up the lookup time for native functions.
   hasProp: -> '{}.hasOwnProperty'

src/nodes.coffee

@@ -897,3 +897,21 @@ test "#4566: destructuring with nested default values", ->
 
   {e: {f = 5} = {}} = {}
   eq 5, f
+
+test "#4674: _extends utility for object spreads 1", ->
+  eqJS(
+    "{a, b..., c..., d}"
+    """
+      var _extends = Object.assign || function (target) { for (var i = 1; i < arguments.length; i++) { var source = arguments[i]; for (var key in source) { if (Object.prototype.hasOwnProperty.call(source, key)) { target[key] = source[key]; } } } return target; };
+
+      _extends({a}, b, c, {d});
+    """
+  )
+
+test "#4674: _extends utility for object spreads 2", ->
+  _extends = -> 3
+  a = b: 1
+  c = d: 2
+  e = {a..., c...}
+  eq e.b, 1
+  eq e.d, 2