Cast operator (.as<...>) in expression language prevents errors reporting

[Mingun]

Take the following KSY:

meta:
  id: test_type
instances:
  me:
    value: unknown.as<u1>

When .as<u1> part is missing, the error is reported, as it should:

test_type: /instances/me/value:
	error: unable to access 'unknown' in test_type context

... but when it is present, that code is generated:

function TestType(_io, _parent, _root) {
  this._io = _io;
  this._parent = _parent;
  this._root = _root || this;
  this._debug = {};

}
TestType.prototype._read = function() {
}
Object.defineProperty(TestType.prototype, 'me', {
  get: function() {
    if (this._m_me !== undefined)
      return this._m_me;
    this._debug._m_me = {  };
    this._m_me = this.unknown;
    return this._m_me;
  }
});

I checked several fields where expressions is allowed and the same behavior is observed in each case.

[generalmimon]

#696 is related.

However, it's true that even without #696 the expression could be checked for at least basic errors such as references to unknown fields.