[modules] Improve error messages.

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/zone-containers.h"
@@ skipping 55 matching lines @@
   void AddStarExport(
     const AstRawString* module_request, const Scanner::Location loc,
     Zone* zone);
 
   // Check if module is well-formed and report error if not.
   // Also canonicalize indirect exports.
   bool Validate(ModuleScope* module_scope,
                 PendingCompilationErrorHandler* error_handler, Zone* zone);
 
   struct Entry : public ZoneObject {
-    const Scanner::Location location;
+    Scanner::Location location;
     const AstRawString* export_name;
     const AstRawString* local_name;
     const AstRawString* import_name;
     // The module_request value records the order in which modules are
     // requested. It also functions as an index into the ModuleInfo's array of
     // module specifiers and into the Module's array of requested modules.  A
     // negative value means no module request.
     int module_request;
 
     // TODO(neis): Remove local_name component?
@@ skipping 111 matching lines @@
                   .insert(std::make_pair(specifier, module_requests_.size()))
                   .first;
     return it->second;
   }
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_AST_MODULES_H_