Address review comments of https://codereview.chromium.org/11299020/

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 15176)
+++ runtime/vm/object.h	(working copy)
@@ -605,6 +605,9 @@
   // Check if this class represents the 'Object' class.
   bool IsObjectClass() const { return id() == kInstanceCid; }
 
+  // Check if this class represents the 'List' class.
+  bool IsListClass() const;
+
   // Check if this class represents a signature class.
   bool IsSignatureClass() const {
     return signature_function() != Object::null();