Compile package:js.{rest, spread} helpers

test/codegen/expect/js/src/varargs.js

+dart_library.library('js/src/varargs', null, /* Imports */[
+  "dart/_runtime",
+  'dart/core'
+], /* Lazy imports */[
+], function(exports, dart, core) {
+  'use strict';
+  let dartx = dart.dartx;
+  class _Rest extends core.Object {
+    _Rest() {
+    }
+  }
+  dart.setSignature(_Rest, {
+    constructors: () => ({_Rest: [_Rest, []]})
+  });
+  let rest = dart.const(new _Rest());
+  function spread(args) {
+    dart.throw(new core.StateError('The spread function cannot be called, ' + 'it should be compiled away.'));
+  }
+  dart.fn(spread);
+  // Exports:
+  exports.rest = rest;
+  exports.spread = spread;
+});