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

src/ast/modules.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_AST_MODULES_H_
 #define V8_AST_MODULES_H_
 
 #include "src/parsing/scanner.h"  // Only for Scanner::Location.
 #include "src/pending-compilation-error-handler.h"
 #include "src/zone-containers.h"
@@ skipping 133 matching lines @@
     special_imports_.Add(entry, zone);
   }
 
  private:
   // TODO(neis): Use STL datastructure instead of ZoneList?
   ZoneList<const Entry*> special_exports_;
   ZoneList<const Entry*> special_imports_;
   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.
   //
   // An explicitly indirect export is an export entry arising from an export
   // statement of the following form:
   //   export {a as c} from "X";
   // An implicitly indirect export corresponds to
   //   export {b as c};
   // in the presence of an import statement of the form
   //   import {a as b} from "X";
   // This function finds such implicitly indirect export entries and rewrites
   // them by filling in the import name and module request, as well as nulling
   // out the local name.  Effectively, it turns
   //   import {a as b} from "X"; export {b as c};
   // into:
   //   import {a as b} from "X"; export {a as c} from "X";
   // (The import entry is never deleted.)
   void MakeIndirectExportsExplicit(Zone* zone);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_AST_MODULES_H_