Textfield _refreshTextLineMetrics seems to break when debugging in Firefox

[mayorbyrne]

Somewhere between Dart 2.3 / Webdev 1.0.1 and the latest Dart 2.6.2 / Webdev 2.5.2, calling almost every getter on a TextField seems to throw an error when debugging in Firefox. Other browsers may be affected, but I haven't tested.

The following code will toss an error when running locally in Firefox with later versions of the SDK/WebDev:

  TextField text = new TextField("hello world");
  print(text.width);

If and when a stack trace is provided (mostly it just appears as Error:), it appears to reference _refreshTextLineMetrics text_field.dart:327

Reverting the SDK and WebDev appears to run the above code without error.