message: STYLE.md: document method for testing with private constructors
diff: @@ -94,6 +94,13 @@ This section describes guidelines for designing public APIs. It can also be appl
  /* ... */
  }
  + // In order to make a private constructor available for testing you can use a
  + // static factory marked as `@internal`, which will not show up in the public API.
  + /** @internal */
  + static forTesting(internalOptions?: { ... }) {
  + return new DefaultImageLoader(internalOptions);
  + }
  +
  private constructor(/* ... */) {
  /* ... */
  }
  