[Spellcheck][Bug] - Immutability violation

[matthew-carroll]

We have some kind of mutability issue in the spellcheck styler.

For example, a client app had a crash within spelling_and_grammar.dart within _applyErrors() where we try to do the following:

viewModel.grammarErrors
  ..clear()
  ..addAll([
    //....
  ]);

Either we want this list to be immutable, in which case we should replace it instead of alter it.

Or, we want this list to be mutable, in which case we need to lookup where we're assigning an unmodifiable list to this variable.