[kernel] Make fasta call a new constructor on FallThroughError

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

Index: pkg/front_end/lib/src/fasta/messages.dart
diff --git a/pkg/front_end/lib/src/fasta/messages.dart b/pkg/front_end/lib/src/fasta/messages.dart
index 1876ccc16df2d42b44a467aabae21540b2e10ed2..b48926c00e36537e62891a5895b02c633588c8e4 100644
--- a/pkg/front_end/lib/src/fasta/messages.dart
+++ b/pkg/front_end/lib/src/fasta/messages.dart
@@ -81,6 +81,12 @@ Location getLocation(String path, int charOffset) {
       ?.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]