This PR makes code below possible:
```javascript
a = document.createElement("a");
a.rel = "foo";
console.log(a.rel); // print out "foo"
```
---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] There are tests for these changes
Source-Repo: https://github.com/servo/servo
Source-Revision: 2e743c2c56d7841ed9135e72581bf7db5e1092b5