Colocation babel plugin drops class decorators

With an input like

```js
import hbs from 'ember-cli-htmlbars-inline-precompile';
const __COLOCATED_TEMPLATE__ = hbs`{{yield}}`;
export default @dec class {}
```

The class decorator `@dec` gets dropped:

```js
import hbs from 'ember-cli-htmlbars-inline-precompile';
const __COLOCATED_TEMPLATE__ = hbs`{{yield}}`;
export default Ember._setComponentTemplate(__COLOCATED_TEMPLATE__, class {});
```

See example https://astexplorer.net/#/gist/4fb9360091ca6422ca0f9e4ee8ef9505/90046a397d94a8ccf8e97c17fd4f2d0cb867a129

I think all that is required to fix is to pass `defaultExportDeclaration.decorators` as the fourth argument to `t.classExpression()`.