This annotation indicates that return value of the annotated method does
not need to be used. If it is _not_ present on a non-void method, and if
Error Prone's `CheckReturnValue` is active, then calling the method
without using the result is an error. However, we are not enabling
`CheckReturnValue` by default here.
Also update some code that does ignore return values, so that the
returned value is used, if only by assigning it to an unused variable.
* Fix RuntimeTypeAdapterFactory depending on internal Streams class
* Clean up gson-extras project, make it Maven module of gson-parent
* Remove broken test from GraphAdapterBuilderTest
