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

Index: pkg/compiler/lib/src/js_emitter/old_emitter/class_builder.dart
diff --git a/pkg/compiler/lib/src/js_emitter/old_emitter/class_builder.dart b/pkg/compiler/lib/src/js_emitter/old_emitter/class_builder.dart
index 92108cd9dac4d0a8f8c8e045ef3c6a80ccba0419..c0650902bc2fc628cdeeb6589a0ee2ee6d4bbde2 100644
--- a/pkg/compiler/lib/src/js_emitter/old_emitter/class_builder.dart
+++ b/pkg/compiler/lib/src/js_emitter/old_emitter/class_builder.dart
@@ -34,16 +34,27 @@ class ClassBuilder {
     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) {