Re-introduce this as a trivial expression...

src/ast.h

Index: src/ast.h
===================================================================
--- src/ast.h	(revision 3991)
+++ src/ast.h	(working copy)
@@ -931,6 +931,10 @@
     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);
   }