Implement scheduleImmediate for the VM.

runtime/lib/schedule_immediate_patch.dart

Index: runtime/lib/schedule_immediate_patch.dart
diff --git a/runtime/lib/schedule_immediate_patch.dart b/runtime/lib/schedule_immediate_patch.dart
new file mode 100644
index 0000000000000000000000000000000000000000..4938a880d0f63670c77635d637641c07ebabe578
--- /dev/null
+++ b/runtime/lib/schedule_immediate_patch.dart
@@ -0,0 +1,18 @@
+// Copyright (c) 2013, 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.
+
+/* patch */ void scheduleImmediate(void callback()) {
+  if (scheduleImmediateClosure == null) {
+    throw new UnsupportedError("microtasks are not supported");
+  }
+  scheduleImmediateClosure(callback);
+}
+
+typedef void ScheduleImmediateClosure(void callback());
+
+ScheduleImmediateClosure scheduleImmediateClosure;
+
+void setScheduleImmediateClosure(ScheduleImmediateClosure closure) {
+  scheduleImmediateClosure = closure;
+}