First cut of mini dart:html.

tool/input_sdk/lib/js/dart2js/js_dart2js.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Support for interoperating with JavaScript.
  *
  * This library provides access to JavaScript objects from Dart, allowing
  * Dart code to get and set properties, and call methods of JavaScript objects
  * and invoke JavaScript functions. The library takes care of converting
@@ skipping 75 matching lines @@
  *     var jsArray = new JsObject.jsify([1, 2, 3]);
  */
 library dart.js;
 
 import 'dart:collection' show HashMap, ListMixin;
 
 import 'dart:_interceptors' as _interceptors show JSArray;
 import 'dart:_js_helper' show Primitives;
 import 'dart:_foreign_helper' show JS;
 
-final JsObject context = _wrapToDart(JS('', 'dart.global'));
+final _global = JS('', 'dart.global');
+final JsObject context = _wrapToDart(_global);
 
 /**
  * Proxies a JavaScript object to Dart.
  *
  * The properties of the JavaScript object are accessible via the `[]` and
  * `[]=` operators. Methods are callable via [callMethod].
  */
 class JsObject {
   // The wrapped JS object.
   final dynamic _jsObject;
@@ skipping 360 matching lines @@
   var wrapper = JS('', 'function(/*...arguments*/) {'
       '  let args = Array.prototype.map.call(arguments, #);'
       '  return #(#(...args));'
       '}', _convertToDart, _convertToJS, f);
   _dartProxies[wrapper] = f;
   return wrapper;
 }
 
 // converts a Dart object to a reference to a native JS object
 // which might be a DartObject JS->Dart proxy
-Object _convertToDart(o) {
+Object _convertToDart(o, [bool isBrowserType(x)]) {
+  if (isBrowserType == null) isBrowserType = _isBrowserType;
   if (JS('bool', '# == null', o) ||
       JS('bool', 'typeof # == "string"', o) ||
       JS('bool', 'typeof # == "number"', o) ||
       JS('bool', 'typeof # == "boolean"', o) ||
-      _isBrowserType(o)) {
+      isBrowserType(o)) {
     return o;
   } else if (JS('bool', '# instanceof Date', o)) {
     var ms = JS('num', '#.getTime()', o);
     return new DateTime.fromMillisecondsSinceEpoch(ms);
   } else if (o is _DartObject &&
              JS('bool', 'dart.jsobject != dart.realRuntimeType(#)', o)) {
     return o._dartObj;
   } else {
     return _putIfAbsent(_dartProxies, o, _wrapToDart);
   }
@@ skipping 54 matching lines @@
         '  let args = [this];'
         '  for (let arg of arguments) {'
         '    args.push(arg);'
         '  }'
         '  return #(...args);'
         '}',
         f);
     _interopCaptureThisExpando[f] = ret;
   }
   return ret;
-}
+}