A few API changes:

sdk/lib/js/dart2js/js_dart2js.dart

 // Copyright (c) 2013, 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.
 
 library dart.js;
 
+import 'dart:collection' show HashMap;
 import 'dart:_foreign_helper' show JS, DART_CLOSURE_TO_JS;
+import 'dart:_interceptors' show JavaScriptObject, UnknownJavaScriptObject;
 import 'dart:_js_helper' show Primitives, convertDartClosureToJS;
 
 final JsObject context = new JsObject._fromJs(Primitives.computeGlobalThis());
 
-JsObject jsify(dynamic data) => data == null ? null : new JsObject._json(data);
-
-class Callback implements Serializable<JsFunction> {
-  final Function _f; // here to allow capture in closure
-  final bool _withThis; // here to allow capture in closure
-  dynamic _jsFunction;
-
-  Callback._(this._f, this._withThis) {
-    _jsFunction = JS('', r'''
-(function(){
-  var f = #;
-  return function(){
-    return f(this, Array.prototype.slice.apply(arguments));
-  };
-}).apply(this)''', convertDartClosureToJS(_call, 2));
-  }
-
-  factory Callback(Function f) => new Callback._(f, false);
-  factory Callback.withThis(Function f) => new Callback._(f, true);
-
-  _call(thisArg, List args) {
-    final arguments = new List.from(args);
-    if (_withThis) arguments.insert(0, thisArg);
-    final dartArgs = arguments.map(_convertToDart).toList();
-    return _convertToJS(Function.apply(_f, dartArgs));
-  }
-
-  JsFunction toJs() => new JsFunction._fromJs(_jsFunction);
-}
-
-/*
- * TODO(justinfagnani): add tests and make public when we remove Callback.
- *
- * Returns a [JsFunction] that captures its 'this' binding and calls [f]
- * with the value of this passed as the first argument.
- */
-JsFunction _captureThis(Function f) => 
-  new JsFunction._fromJs(_convertDartFunction(f, captureThis: true));
-
 _convertDartFunction(Function f, {bool captureThis: false}) {
   return JS('',
     'function(_call, f, captureThis) {'
       'return function() {'
         'return _call(f, captureThis, this, '
             'Array.prototype.slice.apply(arguments));'
       '}'
     '}(#, #, #)', DART_CLOSURE_TO_JS(_callDartFunction), f, captureThis);
 }
 
 _callDartFunction(callback, bool captureThis, self, List arguments) {
   if (captureThis) {
     arguments = [self]..addAll(arguments);
   }
   var dartArgs = arguments.map(_convertToDart).toList();
   return _convertToJS(Function.apply(callback, dartArgs));
 }
 
 
 class JsObject implements Serializable<JsObject> {
   // The wrapped JS object.
   final dynamic _jsObject;
 
   JsObject._fromJs(this._jsObject) {
+    assert(_jsObject != null);
     // Remember this proxy for the JS object
     _getDartProxy(_jsObject, _DART_OBJECT_PROPERTY_NAME, (o) => this);
   }
 
+  /**
+   * Expert users only:
+   * Use this constructor only if you wish to get access to JS expandos
+   * attached to a WebKit native object such as a Node.
+   * An exception will be thrown if a primitive type is passed in passing one
+   * of these types to this method indicates an error.
+   */
+  factory JsObject.fromDartObject(Object object) {

[alexandre.ardhuin, 2013/10/16 19:32:11]

Rename to `fromBrowserObject`.

[justinfagnani, 2013/10/18 03:19:08]

Done.

+    if (object is num || object is String || object is bool || object == null) {
+      throw new IllegalArgumentException(
+        "object cannot be a num, string, bool, or null");
+    }
+    return new JsObject._fromJs(_convertToJS(object));
+  }
+
+  /**
+   * Converts a json-like [data] to a JavaScript map or array and return a
+   * [JsObject] to it.
+   */
+  factory JsObject.jsify(Object object) {
+    if ((object is! Map) && (object is! Iterable)) {
+      throw new IllegalArgumentException("object must be a Map or Iterable");
+    }
+    return new JsObject._fromJs(_convertDataTree(object));
+  }
+
   // TODO(vsm): Type constructor as Serializable<JsFunction> when
   // dartbug.com/11854 is fixed.
   factory JsObject(constructor, [List arguments]) {
     var constr = _convertToJS(constructor);
     if (arguments == null) {
       return new JsObject._fromJs(JS('', 'new #()', constr));
     }
     // The following code solves the problem of invoking a JavaScript
     // constructor with an unknown number arguments.
     // First bind the constructor to the argument list using bind.apply().
     // The first argument to bind() is the binding of 'this', so add 'null' to
     // the arguments list passed to apply().
     // After that, use the JavaScript 'new' operator which overrides any binding
     // of 'this' with the new instance.
     var args = [null]..addAll(arguments.map(_convertToJS));
     var factoryFunction = JS('', '#.bind.apply(#, #)', constr, constr, args);
     // Without this line, calling factoryFunction as a constructor throws
     JS('String', 'String(#)', factoryFunction);
-    return new JsObject._fromJs(JS('', 'new #()', factoryFunction));
+    // This could return an UnknownJavaScriptObject, or a native
+    // object for which there is an interceptor
+    var jsObj = JS('JavaScriptObject', 'new #()', factoryFunction);
+    // print("UnknownJavaScriptObject: ${jsObj is UnknownJavaScriptObject}");

[alexandre.ardhuin, 2013/10/16 19:32:11]

Remove these comments ?

[justinfagnani, 2013/10/18 03:19:08]

Done.

+    // print("JavaScriptObject: ${jsObj is JavaScriptObject}");
+    // JS('void', 'console.log("jsObj: " + #)', jsObj);
+    return new JsObject._fromJs(jsObj);
   }
 
-  factory JsObject._json(data) => new JsObject._fromJs(_convertDataTree(data));
+  // TODO: handle cycles
+  static _convertDataTree(data) {
+    var _convertedObjects = new HashMap.identity();
 
-  static _convertDataTree(data) {
-    if (data is Map) {
-      final convertedData = JS('=Object', '{}');
-      for (var key in data.keys) {
-        JS('=Object', '#[#]=#', convertedData, key,
-            _convertDataTree(data[key]));
+    _convert(o) {
+      if (_convertedObjects.containsKey(o)) {
+        return _convertedObjects[o];
       }
-      return convertedData;
-    } else if (data is Iterable) {
-      return data.map(_convertDataTree).toList();
-    } else {
-      return _convertToJS(data);
+      if (o is Map) {
+        final convertedMap = JS('=Object', '{}');
+        _convertedObjects[o] = convertedMap;
+        for (var key in o.keys) {
+          JS('=Object', '#[#]=#', convertedMap, key, _convert(o[key]));
+        }
+        return convertedMap;
+      } else if (o is Iterable) {
+        var convertedList = [];
+        _convertedObjects[o] = convertedList;
+        convertedList.addAll(o.map(_convert));
+        return convertedList;
+      } else {
+        return _convertToJS(o);
+      }
     }
+
+    return _convert(data);
   }
 
   JsObject toJs() => this;
 
   /**
    * Returns the value associated with [key] from the proxied JavaScript
    * object.
    *
    * [key] must either be a [String] or [int].
    */
@@ skipping 56 matching lines @@
       throw new ArgumentError("name is not a String or int");
     }
     return _convertToDart(JS('', '#[#].apply(#, #)', _jsObject, name,
         _jsObject,
         args == null ? null : args.map(_convertToJS).toList()));
   }
 }
 
 class JsFunction extends JsObject implements Serializable<JsFunction> {
 
+  /*
+   * Returns a [JsFunction] that captures its 'this' binding and calls [f]
+   * with the value of this passed as the first argument.
+   */
+  factory JsFunction.withThis(Function f) {
+    var jsFunc = _convertDartFunction(f, captureThis: true);
+    return new JsFunction._fromJs(jsFunc);
+  }
+
   JsFunction._fromJs(jsObject) : super._fromJs(jsObject);
 
   dynamic apply(thisArg, [List args]) =>
       _convertToDart(JS('', '#.apply(#, #)', _jsObject,
           _convertToJS(thisArg),
           args == null ? null : args.map(_convertToJS).toList()));
 }
 
 abstract class Serializable<T> {
   T toJs();
@@ skipping 71 matching lines @@
 }
 
 Object _getDartProxy(o, String propertyName, createProxy(o)) {
   var dartProxy = JS('', '#[#]', o, propertyName);
   if (dartProxy == null) {
     dartProxy = createProxy(o);
     _defineProperty(o, propertyName, dartProxy);
   }
   return dartProxy;
 }