Add support for throwing exceptions on get(), update() and delete()

[mrunix00]

Fixes #272

[atn832]

Thanks for the PR @mrunix00! Interestingly, it works even on a different reference:

// Works on the same reference.
expect(() => doc.set({'name': 'Bob'}), throwsA(isA<FirebaseException>()));
// Works on a different reference of the same document.
expect(() => instance.collection('users').doc(uid).set({'name': 'Bob'}),
  throwsA(isA<FirebaseException>()));

It's probably luck and how Object.hashcode is implemented. Just to be sure, I'll explicitly implement hashcode in a separate PR.

[atn832]

However, it does not work for Queries:

      final movies = firestore.collection('movies');
      final query = firestore
          .collection('movies')
          .where('title', isEqualTo: 'Test Movie');

      await movies.add({
        'title': 'Test Movie',
      });

      whenCalling(Invocation.method(#get, null))
          .on(query)
          .thenThrow(FirebaseException(plugin: 'firestore'));

      // this throws the exception correctly.
      whenCalling(Invocation.method(#get, null))
          .on(query)
          .thenThrow(FirebaseException(plugin: 'firestore'));

      // but this does not throw the exception.
      expect(
        () async => await firestore
            .collection('movies')
            .where('title', isEqualTo: 'Test Movie')
            .get(GetOptions(source: Source.server)),
        throwsA(isA<FirebaseException>()),
      );

[atn832]

Just uploaded your PR to version 2.4.3. Thanks again! https://pub.dev/packages/fake_cloud_firestore/changelog#243

[mrunix00]

However, it does not work for Queries:

      final movies = firestore.collection('movies');
      final query = firestore
          .collection('movies')
          .where('title', isEqualTo: 'Test Movie');

      await movies.add({
        'title': 'Test Movie',
      });

      whenCalling(Invocation.method(#get, null))
          .on(query)
          .thenThrow(FirebaseException(plugin: 'firestore'));

      // this throws the exception correctly.
      whenCalling(Invocation.method(#get, null))
          .on(query)
          .thenThrow(FirebaseException(plugin: 'firestore'));

      // but this does not throw the exception.
      expect(
        () async => await firestore
            .collection('movies')
            .where('title', isEqualTo: 'Test Movie')
            .get(GetOptions(source: Source.server)),
        throwsA(isA<FirebaseException>()),
      );

Yeah I just noticed that lately, I'll try to find a good fix for that

[atn832]

I debugged a bit earlier, and it seemed that when we call where, it creates an operator function, and this function has a different hash every time even if it has the same parameters. Perhaps if we set up the operator function to return consistent hashes, it will work.