Bruce's example parses some customer data into a Java Customer object. Here's the source from the article, with an additional "parse" factory method that parses a Customer object from a String containing JSON.
testParsesFromJson passes in a String containing JSON, which is what a remote service might do after receiving a response from a server. Notice how I'm not testing getFullName again in this test; it's already been verified in another test.
One challenge with using overlay types is that your objects contain JSNI -- which means that they fail to run in a plain JUnit TestCase. Instead, they must be run in a GWTTestCase, and that means a much slower test.
To solve this problem, I'm tempted to create an interface for the methods I need on my Customer object, then create a Test Double to use in my tests. This would encourage referring to the object by it's interface, which is a good thing.
Congratulations to the GWT team on their hard work in making some great improvements to GWT 1.5!