Revert "Zone support for Futures."

sdk/lib/async/timer.dart

Index: sdk/lib/async/timer.dart
diff --git a/sdk/lib/async/timer.dart b/sdk/lib/async/timer.dart
index 3544728ece9ad5f242f97da7d80a2104ca8e89a9..bfa7ced29b445ca1f0dba4f742b1a339203b8282 100644
--- a/sdk/lib/async/timer.dart
+++ b/sdk/lib/async/timer.dart
@@ -29,9 +29,7 @@ abstract class Timer {
    * Note: If Dart code using Timer is compiled to JavaScript, the finest
    * granularity available in the browser is 4 milliseconds.
    */
-  factory Timer(Duration duration, void callback()) {
-    return _Zone.current.createTimer(duration, callback);
-  }
+  external factory Timer(Duration duration, void callback());
 
   /**
    * Creates a new repeating timer.
@@ -39,10 +37,8 @@ abstract class Timer {
    * The [callback] is invoked repeatedly with [duration] intervals until
    * canceled. A negative duration is treated similar to a duration of 0.
    */
-  factory Timer.periodic(Duration duration,
-                                  void callback(Timer timer)) {
-    return _Zone.current.createPeriodicTimer(duration, callback);
-  }
+  external factory Timer.periodic(Duration duration,
+                                  void callback(Timer timer));
 
   /**
    * Runs the given [callback] asynchronously as soon as possible.
@@ -58,7 +54,3 @@ abstract class Timer {
    */
   void cancel();
 }
-
-external Timer _createTimer(Duration duration, void callback());
-external Timer _createPeriodicTimer(Duration duration,
-                                    void callback(Timer timer));