Address comments from CL 23304012.

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

 // Copyright (c) 2012, 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 js_backend;
 
+/// Enables debugging of fast/slow objects using V8-specific primitives.
 const DEBUG_FAST_OBJECTS = false;
 
 /**
  * A function element that represents a closure call. The signature is copied
  * from the given element.
  */
 class ClosureInvocationElement extends FunctionElementX {
   ClosureInvocationElement(SourceString name,
                            FunctionElement other)
       : super.from(name, other, other.enclosingElement),
@@ skipping 679 matching lines @@
 
     List<jsAst.Node> statements = [
       js('var pendingClasses = {}'),
       js.if_('!init.allClasses', js('init.allClasses = {}')),
       js('var allClasses = init.allClasses'),
 
       js('var hasOwnProperty = Object.prototype.hasOwnProperty'),
 
       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)', [
           js('var desc = collectedClasses[cls]'),
           js('var globalObject = isolateProperties'),
           js.if_('desc instanceof Array', [
               js('globalObject = desc[0] || isolateProperties'),
               js('desc = desc[1]')
           ]),
 
@@ skipping 3006 matching lines @@
           buffer.write(metadata);
         }
       } else {
         throw 'Unexpected value in metadata: ${Error.safeToString(metadata)}';
       }
       buffer.write(',$n');
     }
     buffer.write('];$n');
   }
 
-  void emitInstancify() {
+  void emitConvertToFastObjectFunction() {
     mainBuffer.add(r'''
-function instancify(properties) {
+function convertToFastObject(properties) {
   function makeConstructor() {
     var str = "{\n";
     var hasOwnProperty = Object.prototype.hasOwnProperty;
     for (var property in properties) {
       if (hasOwnProperty.call(properties, property)) {
         str += "this." + property + "= properties." + property + ";\n";
       }
     }
     str += "}\n";
     return new Function("properties", str);
@@ skipping 13 matching lines @@
       computeNeededClasses();
 
       mainBuffer.add(buildGeneratedBy());
       addComment(HOOKS_API_USAGE, mainBuffer);
 
       if (!areAnyElementsDeferred) {
         mainBuffer.add('(function(${namer.CURRENT_ISOLATE})$_{$n');
       }
 
       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 "convertToFastObject" (see
+        // [emitConvertToFastObjectFunction]).
         mainBuffer
             ..write('var ${globalObject}$_=$_{}$N')
             ..write('delete ${globalObject}.x$N');
       }
 
       mainBuffer.add('function ${namer.isolateName}()$_{}\n');
       mainBuffer.add('init()$N$n');
       // Shorten the code by using [namer.CURRENT_ISOLATE] as temporary.
       isolateProperties = namer.CURRENT_ISOLATE;
       mainBuffer.add(
@@ skipping 208 matching lines @@
 
       isolateProperties = isolatePropertiesName;
       // The following code should not use the short-hand for the
       // initialStatics.
       mainBuffer.add('${namer.CURRENT_ISOLATE}$_=${_}null$N');
 
       emitFinishIsolateConstructorInvocation(mainBuffer);
       mainBuffer.add(
           '${namer.CURRENT_ISOLATE}$_=${_}new ${namer.isolateName}()$N');
 
-      emitInstancify();
+      emitConvertToFastObjectFunction();
       for (String globalObject in Namer.reservedGlobalObjectNames) {
-        mainBuffer.add('$globalObject = instancify($globalObject)$N');
+        mainBuffer.add('$globalObject = convertToFastObject($globalObject)$N');
       }
       if (DEBUG_FAST_OBJECTS) {
         ClassElement primitives =
             compiler.findHelper(const SourceString('Primitives'));
         FunctionElement printHelper =
             compiler.lookupElementIn(
                 primitives, const SourceString('printString'));
         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)
                    + ", fast properties " + %HasFastProperties(H));
   $printHelperName("Size of global platform object: "
                    + String(Object.getOwnPropertyNames(P).length)
                    + ", fast properties " + %HasFastProperties(P));
   $printHelperName("Size of global dart:html object: "
                    + String(Object.getOwnPropertyNames(W).length)
                    + ", fast properties " + %HasFastProperties(W));
@@ skipping 274 matching lines @@
 
 const String HOOKS_API_USAGE = """
 // The code supports the following hooks:
 // dartPrint(message)   - if this function is defined it is called
 //                        instead of the Dart [print] method.
 // dartMainRunner(main) - if this function is defined, the Dart [main]
 //                        method will not be invoked directly.
 //                        Instead, a closure that will invoke [main] is
 //                        passed to [dartMainRunner].
 """;