Parameter validation for Async methods

[bgrainger]

There are two general approaches for parameter validation (e.g., ArgumentNullException) with *Async methods:

1. "Eagerly" throw the exception before the Task is created; this requires a non-async method that calls the async helper.
2. Return a faulted Task holding the ArgumentException; this happens by default if parameter checking happens in an async method.

Test if ADO.NET providers differ in how they implement this.