[modules] Improve error messages.

src/ast/modules.h

Index: src/ast/modules.h
diff --git a/src/ast/modules.h b/src/ast/modules.h
index 4d36735fa804f1ed2b5f1ef89067fe1bc57c7e93..dcb5132bd59fa5a1d6eb9ab372429389de6aaef4 100644
--- a/src/ast/modules.h
+++ b/src/ast/modules.h
@@ -73,7 +73,7 @@ class ModuleDescriptor : public ZoneObject {
                 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;