[kernel] Make fasta call a new constructor on FallThroughError

pkg/front_end/lib/src/fasta/messages.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.messages;
 
 import 'package:kernel/ast.dart' show Location;
 
 import 'util/relativize.dart' show relativizeUri;
 
@@ skipping 63 matching lines @@
   } else {
     return message;
   }
 }
 
 Location getLocation(String path, int charOffset) {
   return CompilerContext.current.uriToSource[path]
       ?.getLocation(path, charOffset);
 }
 
+Location getLocationFromUri(Uri uri, int charOffset) {
+  if (charOffset == -1) return null;
+  String path = relativizeUri(uri);
+  return getLocation(path, charOffset);
+}
+
 String getSourceLine(Location location) {
   if (location == null) return null;
   return CompilerContext.current.uriToSource[location.file]
       ?.getTextLine(location.line);
 }