Update `@factory` to allow for statics and nulls.

pkg/meta/CHANGELOG.md

+## 1.0.1
+* Updated `@factory` to allow statics and methods returning `null`.
+
 ## 1.0.0
 * First stable API release.
 
 ## 0.12.2
 * Updated `@protected` to include implemented interfaces (linter#252).
 
 ## 0.12.1
 * Fixed markdown in dartdocs.
 
 ## 0.12.0
 * Introduce `@optionalTypeArgs` annotation for classes whose type arguments are to be treated as optional.
 
 ## 0.11.0
 * Added new `Required` constructor with a means to specify a reason to explain why a parameter is required.
 
 ## 0.10.0
 * Introduce `@factory` annotation for methods that must either be abstract or
 must return a newly allocated object.
 * Introduce `@literal` annotation that indicates that any invocation of a
 constructor must use the keyword `const` unless one or more of the
 arguments to the constructor is not a compile-time constant.
 
 ## 0.9.0
 * Introduce `@protected` annotation for members that must only be called from
 instance members of subclasses.
 * Introduce `@required` annotation for optional parameters that should be treated
 as required.
 * Introduce `@mustCallSuper` annotation for methods that must be invoked by all
 overriding methods.