Make `ErrorWidget`'s text selectable

# Problem

When an assertion is triggered, it is displayed using the `ErrorWidget`.

Unfortunately, its text is unselectable - I cannot copy it to search the internet for help.

# Solution

Ideas:

1. Make the `ErrorWidget`'s text selectable
2. Add a link to Google that searches for the error

# Example scenario

This app has a bad container that provides both a `color` and a `decoration` in a `Container`:

```dart
import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return Container(
color: Colors.blue,
decoration: BoxDecoration(
color: Colors.blue,
),
);
}
}
```

Result: