message: Convert a few more types to ReadonlyArray
  A useful, but somewhat viral annotation.
diff: @@ -74,9 +74,11 @@ export class CodeUnderliner {
  * writing tests simple and legible.
  */
  async underline(reference: Reference): Promise<string>;
  - async underline(references: Reference[]): Promise<string[]>;
  - async underline(reference: Reference|Reference[]): Promise<string|string[]> {
  - if (Array.isArray(reference)) {
  + async underline(references: ReadonlyArray<Reference>):
  + Promise<ReadonlyArray<string>>;
  + async underline(reference: Reference|ReadonlyArray<Reference>):
  + Promise<string|ReadonlyArray<string>> {
  + if (isReadonlyArray(reference)) {
  return Promise.all(reference.map((ref) => this.underline(ref)));
  }
  @@ -92,6 +94,10 @@ export class CodeUnderliner {
  }
  }
  +function isReadonlyArray(maybeArr: any): maybeArr is ReadonlyArray<any> {
  + return Array.isArray(maybeArr);
  +}
  +
  function isWarning(wOrS: Warning|SourceRange): wOrS is Warning {
  return 'code' in wOrS;
  }
  