Clean up the timer source in the isolate library to be less VM specific.

runtime/lib/isolate_patch.dart

Index: runtime/lib/isolate_patch.dart
===================================================================
--- runtime/lib/isolate_patch.dart	(revision 15985)
+++ runtime/lib/isolate_patch.dart	(working copy)
@@ -127,3 +127,37 @@
 
 patch spawnUri(String uri) native "isolate_spawnUri";
 
+patch class Timer {
+  /* patch */ factory Timer(int milliSeconds, void callback(Timer timer)) {
+    if (_TimerFactory._factory == null) {
+      throw new UnsupportedError("Timer interface not supported.");
+    }
+    return _TimerFactory._factory(milliSeconds, callback, false);
+  }
+
+  /**
+   * Creates a new repeating timer. The [callback] is invoked every
+   * [milliSeconds] millisecond until cancelled.
+   */
+  /* patch */ factory Timer.repeating(int milliSeconds,
+                                      void callback(Timer timer)) {
+    if (_TimerFactory._factory == null) {
+      throw new UnsupportedError("Timer interface not supported.");
+    }
+    return _TimerFactory._factory(milliSeconds, callback, true);
+  }
+}
+
+typedef Timer _TimerFactoryClosure(int milliSeconds,
+                                   void callback(Timer timer),
+                                   bool repeating);
+
+class _TimerFactory {
+  static _TimerFactoryClosure _factory;
+}
+
+// TODO(ahe): Warning: this is NOT called by Dartium. Instead, it sets
+// [_TimerFactory._factory] directly.
+void _setTimerFactoryClosure(_TimerFactoryClosure closure) {
+  _TimerFactory._factory = closure;
+}