message: perf(router): don't create new serializer every time UrlTree.toString is called
diff: @@ -115,7 +115,7 @@ export class UrlTree {
  /** The fragment of the URL */
  public fragment: string) {}
  - get queryParamMap() {
  + get queryParamMap(): ParamMap {
  if (!this._queryParamMap) {
  this._queryParamMap = convertToParamMap(this.queryParams);
  }
  @@ -123,7 +123,7 @@ export class UrlTree {
  }
  /** @docsNotRequired */
  - toString(): string { return new DefaultUrlSerializer().serialize(this); }
  + toString(): string { return DEFAULT_SERIALIZER.serialize(this); }
  }
  /**
  @@ -294,6 +294,8 @@ export class DefaultUrlSerializer implements UrlSerializer {
  }
  }
  +const DEFAULT_SERIALIZER = new DefaultUrlSerializer();
  +
  export function serializePaths(segment: UrlSegmentGroup): string {
  return segment.segments.map(p => serializePath(p)).join('/');
  }
  