Fixes #460

lib/runtime/dart/js.js

 dart_library.library('dart/js', null, /* Imports */[
   'dart/_runtime',
   'dart/core',
   'dart/collection',
   'dart/_js_helper'
 ], /* Lazy imports */[
 ], function(exports, dart, core, collection, _js_helper) {
   'use strict';
   let dartx = dart.dartx;
   const _global = dart.global;
   dart.defineLazyProperties(exports, {
     get context() {
       return _wrapToDart(_global);
     }
   });
   const _jsObject = Symbol('_jsObject');
   class JsObject extends core.Object {
     _fromJs(jsObject) {
       this[_jsObject] = jsObject;
       dart.assert(this[_jsObject] != null);
     }
     static new(constructor, arguments$) {
       if (arguments$ === void 0) arguments$ = null;
       let ctor = constructor[_jsObject];
       if (arguments$ == null) {
         return _wrapToDart(new ctor());
       }
-      return _wrapToDart(new ctor(...arguments$));
+      let unwrapped = core.List.from(arguments$[dartx.map](_convertToJS));
+      return _wrapToDart(new ctor(...unwrapped));
     }
     static fromBrowserObject(object) {
       if (typeof object == 'number' || typeof object == 'string' || typeof object == 'boolean' || object == null) {
         dart.throw(new core.ArgumentError("object cannot be a num, string, bool, or null"));
       }
       return _wrapToDart(_convertToJS(object));
     }
     static jsify(object) {
       if (!dart.is(object, core.Map) && !dart.is(object, core.Iterable)) {
         dart.throw(new core.ArgumentError("object must be a Map or Iterable"));
@@ skipping 364 matching lines @@
   }
   dart.fn(allowInteropCaptureThis, core.Function, [core.Function]);
   // Exports:
   exports.JsObject = JsObject;
   exports.JsFunction = JsFunction;
   exports.JsArray$ = JsArray$;
   exports.JsArray = JsArray;
   exports.allowInterop = allowInterop;
   exports.allowInteropCaptureThis = allowInteropCaptureThis;
 });