[public-extension-exports] Class methods, properties are allowed to be tagged with @extensions even if class itself is not.

[ben-polinsky]

With namespaces, we do not allow a member to be exported for extensions without the namespace itself being tagged. However, we do not enforce the same rule for classes.

Example:

export class Foo { // should have error similar to namespace, but no error is present
 /*
 * @extensions
 * @public
 */
 public static bar(){ // no error ok, method is properly tagged
  ...
  }
 }