message: refactor(common): replace `Object.assign` with the spread operator
  `Object.assign` is not available in all supported browsers and one had to
  provide a polyfill. This commit replaces `Object.assign` with the spread
  operator (`...`), which TypeScript will transpile to ES5-compatible code.
diff: @@ -172,7 +172,7 @@ function nameCondition(prop: string, len: number): Intl.DateTimeFormatOptions {
  }
  function combine(options: Intl.DateTimeFormatOptions[]): Intl.DateTimeFormatOptions {
  - return (<any>Object).assign({}, ...options);
  + return options.reduce((merged, opt) => ({...merged, ...opt}), {});
  }
  function datePartGetterFactory(ret: Intl.DateTimeFormatOptions): DateFormatterFn {
  