Implement proper module linking.

src/contexts.cc

Index: src/contexts.cc
diff --git a/src/contexts.cc b/src/contexts.cc
index 76784bd704602b955924c6255c8883b013b1e624..7c610aff674ed3e9cb73122b77209a75ddb99fe8 100644
--- a/src/contexts.cc
+++ b/src/contexts.cc
@@ -304,10 +304,14 @@ void Context::ClearOptimizedFunctions() {
 
 
 #ifdef DEBUG
-bool Context::IsBootstrappingOrContext(Object* object) {
+bool Context::IsBootstrappingOrValidContext(Object* object) {
   // During bootstrapping we allow all objects to pass as
   // contexts. This is necessary to fix circular dependencies.
-  return Isolate::Current()->bootstrapper()->IsActive() || object->IsContext();
+  if (Isolate::Current()->bootstrapper()->IsActive()) return true;
+  if (!object->IsContext()) return false;
+  Context* context = Context::cast(object);
+  return context->IsGlobalContext() || context->IsModuleContext() ||
+         !this->IsModuleContext();
 }