UnmodifiableListView object, when set to a List object, does not throw an analysis error

[maryx]

List is not a subclass of UnmodifiableListView, so we shouldn't be able to set a variable defined with type UnmodifiableListView to a List. When we try to set an UnmodifiableListView to a List, the dart analyzer should throw an error or warning.
E.g. this code is currently allowed and builds fine:

import 'dart:collection';

List<String> _list;
UnmodifiableListView<String> get publicList => _list; // should throw error; currently doesn't

The above should instead be written as:

import 'dart:collection';

List<String> _list;
List<String> get publicList => new UnmodifiableListView<String>(_list);

[matanlurey]

I believe this would be covered by implicit-casts:
https://www.dartlang.org/guides/language/analysis-options#specifying-strong-mode

Seems like a duplicate of #30392.

See also: #30397.