message: Mark _prefix as optional
diff: @@ -11,7 +11,7 @@ export function SubscribeToAllLoggers(subscriber: (message: string) => void) {
  export class Logger {
  private _writer: (message: string) => void;
  - private _prefix: string;
  + private _prefix: string | undefined;
  private _indentLevel: number = 0;
  private _indentSize: number = 4;
  @@ -29,7 +29,7 @@ export class Logger {
  this.write(indent);
  }
  - if (this._prefix) {
  + if (this._prefix !== undefined) {
  this.write(`[${this._prefix}] `);
  }
  @@ -49,13 +49,11 @@ export class Logger {
  }
  }
  - public append(message?: string): void {
  - message = message || "";
  + public append(message: string = ""): void {
  this._appendCore(message);
  }
  - public appendLine(message?: string): void {
  - message = message || "";
  + public appendLine(message: string = ""): void {
  this._appendCore(message + '\n');
  this._atLineStart = true;
  }
  