Address comments from CL 23304012.

dart/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart

Index: dart/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart b/dart/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
index c93307282b5a5ec17d010fe493d9dab0fd1f90bd..cd25786e92e51b83d2c35348ee33533762e19361 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
@@ -4,6 +4,7 @@
 
 part of js_backend;
 
+/// Enables debugging of fast/slow objects using V8-specific primitives.
 const DEBUG_FAST_OBJECTS = false;
 
 /**
@@ -703,7 +704,8 @@ class CodeEmitterTask extends CompilerTask {
 
       optional(
           DEBUG_FAST_OBJECTS,
-          js(r'print("Number of classes: " + Object.getOwnPropertyNames($$).length)')),
+          js(r'print("Number of classes: "'
+             ' + Object.getOwnPropertyNames($$).length)')),
 
       js.forIn('cls', 'collectedClasses', [
         js.if_('hasOwnProperty.call(collectedClasses, cls)', [
@@ -3766,6 +3768,10 @@ function instancify(properties) {
       }
 
       for (String globalObject in Namer.reservedGlobalObjectNames) {
+        // The global objects start as so-called "slow objects". For V8, this
+        // means that it won't try to make map transitions as we add properties
+        // to these objects. Later on, we attempt to turn these objects into
+        // fast objects by calling "instancify".

[ngeoffray, 2013/09/13 10:05:54]

Please add that instancify is being defined in [emitInstancify].

[ahe, 2013/09/13 10:09:12]

Done.

         mainBuffer
             ..write('var ${globalObject}$_=$_{}$N')
             ..write('delete ${globalObject}.x$N');
@@ -4007,6 +4013,7 @@ function instancify(properties) {
         String printHelperName = namer.isolateAccess(printHelper);
 
         mainBuffer.add('''
+// The following only works on V8 when run with option "--allow-natives-syntax".
 if (typeof $printHelperName === "function") {
   $printHelperName("Size of global helper object: "
                    + String(Object.getOwnPropertyNames(H).length)