[modules] Improve error messages.

src/ast/modules.cc

Index: src/ast/modules.cc
diff --git a/src/ast/modules.cc b/src/ast/modules.cc
index 2d28d5564b546b01ea1b2ab5bd3eb56b6572b379..6b527de1f568b8c408cdd11d5d6a0abf1fc75aeb 100644
--- a/src/ast/modules.cc
+++ b/src/ast/modules.cc
@@ -88,7 +88,8 @@ Handle<ModuleInfoEntry> ModuleDescriptor::Entry::Serialize(
       isolate, ToStringOrUndefined(isolate, export_name),
       ToStringOrUndefined(isolate, local_name),
       ToStringOrUndefined(isolate, import_name),
-      Handle<Object>(Smi::FromInt(module_request), isolate));
+      Handle<Object>(Smi::FromInt(module_request), isolate),
+      location.beg_pos, location.end_pos);
 }
 
 ModuleDescriptor::Entry* ModuleDescriptor::Entry::Deserialize(
@@ -184,6 +185,14 @@ void ModuleDescriptor::MakeIndirectExportsExplicit(Zone* zone) {
                 static_cast<int>(module_requests_.size()));
       entry->import_name = import->second->import_name;
       entry->module_request = import->second->module_request;
+      // Hack: When the indirect export cannot be resolved, we want the error
+      // message to point at the import statement, not at the export statement.
+      // Therefore we overwrite [entry]'s location here.  Note that Validate()
+      // has already checked for duplicate exports, so it's guaranteed that we
+      // won't need to report any error pointing at the (now lost) export
+      // location.
+      entry->location.beg_pos = import->second->location.beg_pos;

[adamk, 2016/10/26 12:38:16]

Why can't you just assign the location directly?

+      entry->location.end_pos = import->second->location.end_pos;
       entry->local_name = nullptr;
       AddSpecialExport(entry, zone);
       it = regular_exports_.erase(it);