Move core and coreimpl from corelib/ to lib/core and lib/coreimpl.

corelib/src/stopwatch.dart

-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-/**
- * A simple [Stopwatch] interface to measure elapsed time.
- */
-interface Stopwatch default StopwatchImplementation {
-
-  /**
-   * Creates a [Stopwatch] in stopped state with a zero elapsed count.
-   */
-  Stopwatch();
-
-  /**
-   * Creates a [Stopwatch] in started state with a zero elapsed count.
-   */
-  Stopwatch.start();
-
-  /**
-   * 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.
-   * If the [Stopwatch] is currently running, then calling start does nothing.
-   */
-  void start();
-
-  /**
-   * Stops the [Stopwatch]. The [elapsed] count stops increasing.
-   * If the [Stopwatch] is currently not running, then calling stop does
-   * nothing.
-   */
-  void stop();
-
-  /**
-   * Resets the [elapsed] count to zero. This method does not stop or start
-   * the [Stopwatch].
-   */
-  void reset();
-
-  /**
-   * Returns the elapsed number of clock ticks since calling [start] while the
-   * [Stopwatch] is running.
-   * Returns the elapsed number of clock ticks between calling [start] and
-   * calling [stop].
-   * Returns 0 if the [Stopwatch] has never been started.
-   * The elapsed number of clock ticks increases by [frequency] every second.
-   */
-  int elapsed();
-
-  /**
-   * Returns the [elapsed] counter converted to microseconds.
-   */
-  int elapsedInUs();
-
-  /**
-   * Returns the [elapsed] counter converted to milliseconds.
-   */
-  int elapsedInMs();
-
-  /**
-   * Returns the frequency of the elapsed counter in Hz.
-   */
-  int frequency();
-
-}