The name `FakeIdentityManagementService` makes it sound like it might be used as a [test double](https://en.wikipedia.org/wiki/Test_Double).

Now that I understand what this class does and how/when it is used, I would prefer a more descriptive name, such as `SimpleIdentityManagementService` or, more appropriately, `LegacyIdentityManagementService`, with a code documentation comment explaining that this class is used when the TFS server runs a version older than 2010.

Even though, at the moment, this implementation of `IIdentityManagementService` doesn't allow callers to resolve users' e-mail addresses or their display name, someone may eventually add that feature.