Implement class hierarchy analysis in the VM.

runtime/vm/class_finalizer.cc

Index: runtime/vm/class_finalizer.cc
===================================================================
--- runtime/vm/class_finalizer.cc	(revision 11005)
+++ runtime/vm/class_finalizer.cc	(working copy)
@@ -1122,6 +1122,9 @@
                   name.ToCString());
     }
     cls.Finalize();
+    // Signature classes extend Object. No need to add this class to the direct
+    // subclasses of Object.
+    ASSERT(super_type.IsNull() || super_type.IsObjectType());
     return;
   }
   // Finalize factory class, if any.
@@ -1168,6 +1171,12 @@
                   cls_name.ToCString(), lib_name.ToCString());
     }
   }
+  // Add this class to the direct subclasses of the superclass, unless the
+  // superclass is Object.
+  if (!super_type.IsNull() && !super_type.IsObjectType()) {
+    ASSERT(!super_class.IsNull());
+    super_class.AddDirectSubclass(cls);
+  }
 }