Fix the semantics of delete on parameters.

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index c3dfd5ff261db65fa7ad2c1d148253ed4baa9967..d599bd6f8b2c3cf16490a38ce2137bb39d066c8f 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -4653,12 +4653,18 @@ void HGraphBuilder::VisitUnaryOperation(UnaryOperation* expr) {
       // The subexpression does not have side effects.
       ast_context()->ReturnValue(graph()->GetConstantFalse());
     } else if (prop != NULL) {
-      VISIT_FOR_VALUE(prop->obj());
-      VISIT_FOR_VALUE(prop->key());
-      HValue* key = Pop();
-      HValue* obj = Pop();
-      ast_context()->ReturnInstruction(new HDeleteProperty(obj, key),
-                                       expr->id());
+      if (prop->is_synthetic()) {
+        // Result of deleting parameters is false, even when they rewrite
+        // to accesses on the arguments object.
+        ast_context()->ReturnValue(graph()->GetConstantFalse());
+      } else {
+        VISIT_FOR_VALUE(prop->obj());
+        VISIT_FOR_VALUE(prop->key());
+        HValue* key = Pop();
+        HValue* obj = Pop();
+        HDeleteProperty* instr = new HDeleteProperty(obj, key);
+        ast_context()->ReturnInstruction(instr, expr->id());
+      }
     } else if (var->is_global()) {
       BAILOUT("delete with global variable");
     } else {