Reland of "[modules] Make duplicate export error deterministic."

src/ast/modules.h

Index: src/ast/modules.h
diff --git a/src/ast/modules.h b/src/ast/modules.h
index 0ed3509bc1b9ecfd6a8b88bf2ef0576dfff2dba3..bdc31b24e630c288fc6eb1f732270797f85f7ee9 100644
--- a/src/ast/modules.h
+++ b/src/ast/modules.h
@@ -151,8 +151,8 @@ class ModuleDescriptor : public ZoneObject {
   ZoneMultimap<const AstRawString*, Entry*> regular_exports_;
   ZoneMap<const AstRawString*, const Entry*> regular_imports_;
 
-  // If there are multiple export entries with the same export name, return one
-  // of them.  Otherwise return nullptr.
+  // If there are multiple export entries with the same export name, return the
+  // last of them (in source order).  Otherwise return nullptr.
   const Entry* FindDuplicateExport(Zone* zone) const;
 
   // Find any implicitly indirect exports and make them explicit.