Undefined is not a literal.

src/ast.cc

Index: src/ast.cc
diff --git a/src/ast.cc b/src/ast.cc
index 964f5bc76e2ea88b9b0f16fac9f167ed3d5bd927..f34c7bb24aac7d500016acb4d62a846eadf82ce1 100644
--- a/src/ast.cc
+++ b/src/ast.cc
@@ -71,8 +71,14 @@ bool Expression::IsNullLiteral() {
 }
 
 
-bool Expression::IsUndefinedLiteral() {
-  return AsLiteral() != NULL && AsLiteral()->value()->IsUndefined();
+bool Expression::IsUndefinedLiteral(Isolate* isolate) {
+  VariableProxy* var_proxy = AsVariableProxy();
+  if (var_proxy == NULL) return false;
+  Variable* var = var_proxy->var();
+  // The global identifier "undefined" is immutable. Everything
+  // else could be reassigned.
+  return var != NULL && var->location() == Variable::UNALLOCATED &&
+         var_proxy->name()->Equals(isolate->heap()->undefined_string());
 }
 
 
@@ -385,12 +391,13 @@ static bool IsVoidOfLiteral(Expression* expr) {
 static bool MatchLiteralCompareUndefined(Expression* left,
                                          Token::Value op,
                                          Expression* right,
-                                         Expression** expr) {
+                                         Expression** expr,
+                                         Isolate* isolate) {
   if (IsVoidOfLiteral(left) && Token::IsEqualityOp(op)) {
     *expr = right;
     return true;
   }
-  if (left->IsUndefinedLiteral() && Token::IsEqualityOp(op)) {
+  if (left->IsUndefinedLiteral(isolate) && Token::IsEqualityOp(op)) {
     *expr = right;
     return true;
   }
@@ -398,9 +405,10 @@ static bool MatchLiteralCompareUndefined(Expression* left,
 }
 
 
-bool CompareOperation::IsLiteralCompareUndefined(Expression** expr) {
-  return MatchLiteralCompareUndefined(left_, op_, right_, expr) ||
-      MatchLiteralCompareUndefined(right_, op_, left_, expr);
+bool CompareOperation::IsLiteralCompareUndefined(
+    Expression** expr, Isolate* isolate) {
+  return MatchLiteralCompareUndefined(left_, op_, right_, expr, isolate) ||
+      MatchLiteralCompareUndefined(right_, op_, left_, expr, isolate);
 }