Generate tear-off closures dynamically.

dart/sdk/lib/_internal/compiler/implementation/js_emitter/class_builder.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.
 
 part of dart2js.js_emitter;
 
 /**
  * A data structure for collecting fragments of a class definition.
  */
 class ClassBuilder {
   // TODO (sigurdm) this is just a bag of properties, rename this class
 
   final List<jsAst.Property> properties = <jsAst.Property>[];
+  final List<String> fields = <String>[];
+
+  String superName;
+  String nativeName;
+  String functionType;
+  List<jsAst.Node> fieldMetadata;
 
   /// Set to true by user if class is indistinguishable from its superclass.
   bool isTrivial = false;
 
   // Has the same signature as [DefineStubFunction].
   void addProperty(String name, jsAst.Expression value) {
     properties.add(new jsAst.Property(js.string(name), value));
   }
 
-  jsAst.Expression toObjectInitializer() {
-    return new jsAst.ObjectInitializer(properties, isOneLiner: false);
+  void addField(String field) {
+    fields.add(field);
+  }
+
+  jsAst.ObjectInitializer toObjectInitializer() {
+    StringBuffer buffer = new StringBuffer();
+    if (superName != null) {
+      if (nativeName != null) {
+        buffer.write('$nativeName/');
+      }
+      buffer.write('$superName');
+      if (functionType != null) {
+        buffer.write(':$functionType');
+      }
+      buffer.write(';');
+    }
+    buffer.writeAll(fields, ',');
+    var classData = js.string('$buffer');
+    if (fieldMetadata != null) {
+      classData =
+          new jsAst.ArrayInitializer.from([classData]..addAll(fieldMetadata));
+    }
+    var fieldsAndProperties =
+        [new jsAst.Property(js.string(''), classData)]
+        ..addAll(properties);
+    return new jsAst.ObjectInitializer(fieldsAndProperties, isOneLiner: false);
   }
 
 }