Computed properties with no dependencies generate a syntax error

[justinfagnani]

Given this property definition:

    properties: {
        _documentWindow: {
          type: Object,
          computed: '_getDocumentWindow()',
        },
    }

We generate this code:

(function(old
/**/) {
    this.debounce('__getDocumentWindow()', function() {
        this._documentWindow = this._getDocumentWindow(this._data.);
    });
})

[sorvell]

For this type of use case, you should use value like this:

properties: {
     _documentWindow: {
          type: Object,
          value: function() { return this._getDocumentWindow(); }
      }
}