[Scope] Issue with scope.unwatch() not working

[vicb]

Consider the following TC:

      iit(r'should be possible to remove every watch',
          (RootScope rootScope, FilterMap filters) {
        rootScope.context['foo'] = 'bar';
        var watch1 = rootScope.watch('(foo|json)+"bar"', (v, p) => null,
        filters: filters);
        var watch2 = rootScope.watch('(foo|json)+"bar"', (v, p) => null,
        filters: filters);

        expect(() => watch1.remove()).not.toThrow();
        expect(() => watch2.remove()).not.toThrow();
      });

expect(() => watch2.remove()).not.toThrow(); would throw with Test failed: Caught 'package:angular/change_detection/watch_group.dart': Failed assertion: line 826 pos 12: 'mode != _MODE_DELETED_' is not true.

As we are watching the same expression twice, removing it once removes both. Note: it is not an issue with simple expressions like foo only

[vicb]

Whenever this gets fixed, please unxit this test from #788.