[strong] class objects created in strong mode have their prototype frozen

src/x64/full-codegen-x64.cc

Index: src/x64/full-codegen-x64.cc
diff --git a/src/x64/full-codegen-x64.cc b/src/x64/full-codegen-x64.cc
index 1bcbdfe59999528f1693f2395f0edec2906ea7b4..7e8ae79c9274c0a071b0abaff3243f0630012a34 100644
--- a/src/x64/full-codegen-x64.cc
+++ b/src/x64/full-codegen-x64.cc
@@ -2567,6 +2567,17 @@ void FullCodeGenerator::EmitClassDefineProperties(ClassLiteral* lit,
 
   // constructor
   __ CallRuntime(Runtime::kToFastProperties, 1);
+
+  if (is_strong(language_mode())) {
+    __ movp(scratch,
+            FieldOperand(rax, JSFunction::kPrototypeOrInitialMapOffset));
+    __ Push(rax);
+    __ Push(scratch);
+    __ CallRuntime(Runtime::kObjectFreeze, 1);
+    // TODO(conradw): It would be more efficient to define the properties with
+    // the right attributes the first time round.
+    __ CallRuntime(Runtime::kObjectFreeze, 1);
+  }
 }