closes again1 #849-give-more-useful-error-message-when-sc-file-format…

…-is-incorrect

lib/src/reducers/load_dna_file_reducer.dart

@@ -1,5 +1,5 @@
 import 'dart:convert';
-import 'dart:html';
+//import 'dart:html';
 
 import 'package:built_collection/built_collection.dart';
 import 'package:scadnano/src/dna_file_type.dart';
@@ -39,7 +39,7 @@ The design has the following problem:
 ${error.cause}
 ${util.stack_trace_message_bug_report(stack_trace)}''';
   } catch (error, stack_trace) {
-    window.alert('I was unable to process that file. Only scadnano .sc files are supported for opening via the menu File-->Open or dragging onto the browser. If you are trying to import a cadnano file (ending. in .json), use the menu option File-->Import cadnano v2. Here is the full error message:');
+    //window.alert('I was unable to process that file. Only scadnano .sc files are supported for opening via the menu File-->Open or dragging onto the browser. If you are trying to import a cadnano file (ending. in .json), use the menu option File-->Import cadnano v2. Here is the full error message:');
     error_message = "I encountered an error while reading the file ${action.filename}:"
         '\n\n$hline'
         '\n* error type:    ${error.runtimeType}'