Fix a bug in delete for lookup slots.

src/x64/codegen-x64.cc

Index: src/x64/codegen-x64.cc
diff --git a/src/x64/codegen-x64.cc b/src/x64/codegen-x64.cc
index 57720a8019b362eb03c12747dc2182c91cec0d4f..e13b2e9004929de5c6e6b18043445893a5a3f051 100644
--- a/src/x64/codegen-x64.cc
+++ b/src/x64/codegen-x64.cc
@@ -7235,19 +7235,13 @@ void CodeGenerator::VisitUnaryOperation(UnaryOperation* node) {
         return;
 
       } else if (slot != NULL && slot->type() == Slot::LOOKUP) {
-        // Call the runtime to look up the context holding the named
+        // Call the runtime to delete from the context holding the named
         // variable.  Sync the virtual frame eagerly so we can push the
         // arguments directly into place.
         frame_->SyncRange(0, frame_->element_count() - 1);
         frame_->EmitPush(rsi);
         frame_->EmitPush(variable->name());
-        Result context = frame_->CallRuntime(Runtime::kLookupContext, 2);
-        ASSERT(context.is_register());
-        frame_->EmitPush(context.reg());
-        context.Unuse();
-        frame_->EmitPush(variable->name());
-        Result answer = frame_->InvokeBuiltin(Builtins::DELETE,
-                                              CALL_FUNCTION, 2);
+        Result answer = frame_->CallRuntime(Runtime::kDeleteContextSlot, 2);
         frame_->Push(&answer);
         return;
       }