dart2js: Add a pointer from the emitter's JS code to the encoding part.

pkg/compiler/lib/src/js_emitter/old_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 {
@@ skipping 16 matching lines @@
   jsAst.Property addProperty(String name, jsAst.Expression value) {
     jsAst.Property property = new jsAst.Property(js.string(name), value);
     properties.add(property);
     return property;
   }
 
   void addField(String field) {
     fields.add(field);
   }
 
+  static String functionTypeEncodingDescription =
+      'For simple function types the function type is stored in the metadata '
+      'and the index is encoded into the superclass field.';
+
+  static String fieldEncodingDescription =
+      'Fields are encoded as a comma separated list. If there is a superclass '
+      '(and possibly a function type encoding) the fields are separated from '
+      'the superclass by a semicolon.';
+
   jsAst.ObjectInitializer toObjectInitializer(
       {bool emitClassDescriptor: true}) {
     StringBuffer buffer = new StringBuffer();
     if (superName != null) {
-      buffer.write('$superName');
+      buffer.write(superName);
       if (functionType != null) {
+        // See [functionTypeEncodingDescription] above.
         buffer.write(':$functionType');
       }
       buffer.write(';');
     }
+    // See [fieldEncodingDescription] above.
     buffer.writeAll(fields, ',');
     var classData = js.string('$buffer');
     if (fieldMetadata != null) {
       // If we need to store fieldMetadata, classData is turned into an array,
       // and the field metadata is appended. So if classData is just a string,
       // there is no field metadata.
       classData =
           new jsAst.ArrayInitializer([classData]..addAll(fieldMetadata));
     }
     List<jsAst.Property> fieldsAndProperties;
     if (emitClassDescriptor) {
       fieldsAndProperties = <jsAst.Property>[];
       fieldsAndProperties.add(
           new jsAst.Property(
               js.string(namer.classDescriptorProperty), classData));
       fieldsAndProperties.addAll(properties);
     } else {
       fieldsAndProperties = properties;
     }
     return new jsAst.ObjectInitializer(fieldsAndProperties, isOneLiner: false);
   }
 }