Re-apply 'Unify dart:isolate.' with fixes for autogenerated dart2js and dartium html files.

lib/isolate/timer.dart

Index: lib/isolate/timer.dart
diff --git a/lib/isolate/timer.dart b/lib/isolate/timer.dart
index 95de18eb3b1c12b064f689f8e96bdcb95d2a8cc0..76f98a1c2cd610ed00ffdbb84d6a6c1b65be3363 100644
--- a/lib/isolate/timer.dart
+++ b/lib/isolate/timer.dart
@@ -20,3 +20,34 @@ interface Timer default _TimerFactory {
    */
   void cancel();
 }
+
+// TODO(ajohnsen): Patch timer once we have support for patching named
+//                 factory constructors in the VM.
+
+typedef Timer _TimerFactoryClosure(int milliSeconds,
+                                   void callback(Timer timer),
+                                   bool repeating);
+
+// _TimerFactory provides a hook which allows various implementations of this
+// library to provide a concrete class for the Timer interface.
+class _TimerFactory {
+  factory Timer(int milliSeconds, void callback(Timer timer)) {
+    if (_factory == null) {
+      throw new UnsupportedOperationException("Timer interface not supported.");
+    }
+    return _factory(milliSeconds, callback, false);
+  }
+
+  factory Timer.repeating(int milliSeconds, void callback(Timer timer)) {
+    if (_factory == null) {
+      throw new UnsupportedOperationException("Timer interface not supported.");
+    }
+    return _factory(milliSeconds, callback, true);
+  }
+
+  static _TimerFactoryClosure _factory;
+}
+
+void _setTimerFactoryClosure(_TimerFactoryClosure closure) {
+  _TimerFactory._factory = closure;
+}