Introduce global contexts to represent lexical global scope(s).

src/objects-debug.cc

Index: src/objects-debug.cc
diff --git a/src/objects-debug.cc b/src/objects-debug.cc
index 4343ad1893ff2d9c2e330e3b12564fc73f8c637c..6f02849b6362016cd647efb637d2ef99b27e65c6 100644
--- a/src/objects-debug.cc
+++ b/src/objects-debug.cc
@@ -382,7 +382,8 @@ void FixedDoubleArray::FixedDoubleArrayVerify() {
 void JSModule::JSModuleVerify() {
   VerifyObjectField(kContextOffset);
   VerifyObjectField(kScopeInfoOffset);
-  CHECK(context()->IsUndefined() || context()->IsModuleContext());
+  CHECK(context()->IsUndefined() ||
+        Context::cast(context())->IsModuleContext());
 }