[modules] Basic support of exports

src/ast/variables.h

Index: src/ast/variables.h
diff --git a/src/ast/variables.h b/src/ast/variables.h
index e7f53eba8e78c57b92cdf93f9cd0100faf13d08d..dd99cac678ef8087d7225325c0d807a45be136a1 100644
--- a/src/ast/variables.h
+++ b/src/ast/variables.h
@@ -102,6 +102,11 @@ class Variable final : public ZoneObject {
 
   int index() const { return index_; }
 
+  bool IsExport() const {
+    DCHECK(location() == VariableLocation::MODULE);
+    return index() == 0;
+  }
+
   void AllocateTo(VariableLocation location, int index) {
     DCHECK(IsUnallocated() ||
            (this->location() == location && this->index() == index));