Compile package:js.{rest, spread} helpers

test/codegen/expect/varargs.js

+dart_library.library('varargs', null, /* Imports */[
+  "dart/_runtime"
+], /* Lazy imports */[
+], function(exports, dart) {
+  'use strict';
+  let dartx = dart.dartx;
+  function varargsTest(x, ...others) {
+    let args = [1, others];
+    dart.dsend(x, 'call', ...args);
+  }
+  dart.fn(varargsTest);
+  function varargsTest2(x, ...others) {
+    let args = [1, others];
+    dart.dsend(x, 'call', ...args);
+  }
+  dart.fn(varargsTest2);
+  // Exports:
+  exports.varargsTest = varargsTest;
+  exports.varargsTest2 = varargsTest2;
+});