Don't treat this as a trivial expression....

src/ast.h

Index: src/ast.h
===================================================================
--- src/ast.h	(revision 3938)
+++ src/ast.h	(working copy)
@@ -931,10 +931,6 @@
     return var()->is_global() || var()->rewrite()->IsLeaf();
   }
 
-  // Reading from a mutable variable is a side effect, but 'this' is
-  // immutable.
-  virtual bool IsTrivial() { return is_this(); }
-
   bool IsVariable(Handle<String> n) {
     return !is_this() && name().is_identical_to(n);
   }