Implement rudimentary module linking.

src/objects-debug.cc

Index: src/objects-debug.cc
diff --git a/src/objects-debug.cc b/src/objects-debug.cc
index 8eefb23db280e804ca70212c0ea9f73844a0119b..cd2ccf81c775187266357dc49c371b04ede4e431 100644
--- a/src/objects-debug.cc
+++ b/src/objects-debug.cc
@@ -135,6 +135,9 @@ void HeapObject::HeapObjectVerify() {
     case JS_CONTEXT_EXTENSION_OBJECT_TYPE:
       JSObject::cast(this)->JSObjectVerify();
       break;
+    case JS_MODULE_TYPE:
+      JSModule::cast(this)->JSModuleVerify();
+      break;
     case JS_VALUE_TYPE:
       JSValue::cast(this)->JSValueVerify();
       break;
@@ -366,6 +369,15 @@ void FixedDoubleArray::FixedDoubleArrayVerify() {
 }
 
 
+void JSModule::JSModuleVerify() {
+  Object* v = context();
+  if (v->IsHeapObject()) {
+    VerifyHeapPointer(v);
+  }
+  CHECK(v->IsUndefined() || v->IsModuleContext());
+}
+
+
 void JSValue::JSValueVerify() {
   Object* v = value();
   if (v->IsHeapObject()) {