Port the remaining of dart:coreimpl to the unified corelib.

corelib/unified/coreimpl/stopwatch.dart

Index: corelib/unified/coreimpl/stopwatch.dart
diff --git a/corelib/src/implementation/stopwatch_implementation.dart b/corelib/unified/coreimpl/stopwatch.dart
similarity index 76%
copy from corelib/src/implementation/stopwatch_implementation.dart
copy to corelib/unified/coreimpl/stopwatch.dart
index 1316a27fd9573d051e20c49993e779ba244b898c..581ca561f41661aa32412bb91a9b7fa87e1e9fc6 100644
--- a/corelib/src/implementation/stopwatch_implementation.dart
+++ b/corelib/unified/coreimpl/stopwatch.dart
@@ -22,14 +22,14 @@ class StopwatchImplementation implements Stopwatch {
   void start() {
     if (_start === null) {
       // This stopwatch has never been started.
-      _start = Clock.now();
+      _start = _Clock.now();
     } else {
       if (_stop === null) {
         return;
       }
       // Restarting this stopwatch. Prepend the elapsed time to the current
       // start time.
-      _start = Clock.now() - (_stop - _start);
+      _start = _Clock.now() - (_stop - _start);
       _stop = null;
     }
   }
@@ -38,14 +38,14 @@ class StopwatchImplementation implements Stopwatch {
     if (_start === null || _stop !== null) {
       return;
     }
-    _stop = Clock.now();
+    _stop = _Clock.now();
   }
 
   void reset() {
     if (_start === null) return;
     // If [_start] is not null, then the stopwatch had already been started. It
     // may running right now.
-    _start = Clock.now();
+    _start = _Clock.now();
     if (_stop !== null) {
       // The watch is not running. So simply set the [_stop] to [_start] thus
       // having an elapsed time of 0.
@@ -57,7 +57,7 @@ class StopwatchImplementation implements Stopwatch {
     if (_start === null) {
       return 0;
     }
-    return (_stop === null) ? (Clock.now() - _start) : (_stop - _start);
+    return (_stop === null) ? (_Clock.now() - _start) : (_stop - _start);
   }
 
   int elapsedInUs() {
@@ -69,7 +69,23 @@ class StopwatchImplementation implements Stopwatch {
   }
 
   int frequency() {
-    return Clock.frequency();
+    return _Clock.frequency();
   }
+}
+
+/**
+ * The class [_Clock] provides access to a monotonically incrementing clock
+ * device.
+ */
+class _Clock {
+
+  /**
+   * Returns the current clock tick.
+   */
+  external static int now();
 
+  /**
+   * Returns the frequency of clock ticks in Hz.
+   */
+  external static int frequency();
 }