anchor.diff:  
   part of dart.core;
   
  -/// A simple stopwatch interface to measure elapsed time.
  +/// A stopwatch which measures time while it's running.
  +///
  +/// A stopwatch is either running or stopped.
  +/// It measures the elapsed time that passes while the stopwatch is running.
  +///
  +/// When a stopwatch is initially created, it is stopped and has measured no
  +/// elapsed time.
  +///
  +/// The elapsed time can be accessed in various formats using
  +/// [elapsed], [elapsedMilliseconds], [elapsedMicroseconds] or [elapsedTicks].
  +///
  +/// The stopwatch is started by calling [start].
  +///
  +/// Example:
  +/// ```dart
  +/// final stopwatch = Stopwatch();
  +/// print(stopwatch.elapsedMilliseconds); // 0
  +/// print(stopwatch.isRunning); // false
  +/// stopwatch.start();
  +/// print(stopwatch.isRunning); // true
  +/// ```
  +/// To stop or pause the stopwatch, use [stop].
  +/// Use [start] to continue again when only pausing temporarily.
  +/// ```
  +/// stopwatch.stop();
  +/// print(stopwatch.isRunning); // false
  +/// Duration elapsed = stopwatch.elapsed;
  +/// await Future.delayed(const Duration(seconds: 1));
  +/// assert(stopwatch.elapsed == elapsed); // No measured time elapsed.
  +/// stopwatch.start(); // Continue measuring.
  +/// ```
  +/// The [reset] method sets the elapsed time back to zero.
  +/// It can be called whether the stopwatch is running or not,
  +/// and doesn't change whether it's running.
  +/// ```
  +/// // Do some work.
  +/// stopwatch.stop();
  +/// print(stopwatch.elapsedMilliseconds); // Likely > 0.
  +/// stopwatch.reset();
  +/// print(stopwatch.elapsedMilliseconds); // 0
  +/// ```
   class Stopwatch {
     /// Cached frequency of the system in Hz (ticks per second).
     ///
  
positive.code: 
    /// Starts the [Stopwatch].
    ///
    /// The [elapsed] count is increasing monotonically. If the [Stopwatch] has
    /// been stopped, then calling start again restarts it without resetting the
    /// [elapsed] count.
    ///
  
negative.code:   /// The following example shows how to start a [Stopwatch]
    /// immediately after allocation.
    /// ```dart
    /// final stopwatch = Stopwatch()..start();
    /// ```
    Stopwatch() {
      _frequency; // Ensures initialization before using any method.
  