[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

src/nodes.coffee

@@ -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'