Introduce 'trivial' expressions, use them for this property assignments.

src/ast.h

Index: src/ast.h
diff --git a/src/ast.h b/src/ast.h
index 8e717a6c8c36b0078a8dd43dfe9ab5c2b560fafe..927a9f50354072d37fef719a471c5d13f06ea55e 100644
--- a/src/ast.h
+++ b/src/ast.h
@@ -1,4 +1,4 @@
-// Copyright 2006-2008 the V8 project authors. All rights reserved.
+// Copyright 2010 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
@@ -199,6 +199,10 @@ class Expression: public AstNode {
   // evaluated.
   virtual bool IsLeaf() { return false; }
 
+  // True if the expression has no side effects and is safe to
+  // evaluate out of order.
+  virtual bool IsTrivial() { return false; }
+
   // Mark the expression as being compiled as an expression
   // statement. This is used to transform postfix increments to
   // (faster) prefix increments.
@@ -738,6 +742,7 @@ class Literal: public Expression {
   }
 
   virtual bool IsLeaf() { return true; }
+  virtual bool IsTrivial() { return true; }
 
   // Identity testers.
   bool IsNull() const { return handle_.is_identical_to(Factory::null_value()); }
@@ -926,6 +931,10 @@ class VariableProxy: public Expression {
     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);
   }