Helper to implement a setter of a property with the given name on a
polymer element. This can be used on normal properties and also on
computed properties, as long as the expression used for the computed
property is assignable (see ComputedProperty).
writeValue(Symbol name, newValue) {
var property = _properties[name];
if (property == null) {
// Note: computed properties are created in advance in
// createComputedProperties, so we should only need to create here
// non-computed properties.
property = _properties[name] = new _PropertyAccessor(name, this, null);
}
property.value = newValue;
}