This adds a new class org.apache.hadoop.util.Preconditions which is
* @Private/@Unstable
* Intended to allow us to move off Google Guava
* Is designed to be trivially backportable
(i.e contains no references to guava classes internally)
Please use this instead of the guava equivalents, where possible.
Contributed by: Ahmed Hussein
Change-Id: Ic392451bcfe7d446184b7c995734bcca8c07286e
9188fa8cce