Update headings to use sentence case.

sdk/lib/async/schedule_microtask.dart

 // 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.
 
 part of dart.async;
 
 typedef void _AsyncCallback();
 
 bool _callbacksAreEnqueued = false;
 Queue<_AsyncCallback> _asyncCallbacks = new Queue<_AsyncCallback>();
@@ skipping 28 matching lines @@
  * Runs a function asynchronously.
  *
  * Callbacks registered through this function are always executed in order and
  * are guaranteed to run before other asynchronous events (like [Timer] events,
  * or DOM events).
  *
  * **Warning:** it is possible to starve the DOM by registering asynchronous
  * callbacks through this method. For example the following program runs
  * the callbacks without ever giving the Timer callback a chance to execute:
  *
- *     Timer.run(() { print("executed"); });  // Will never be executed,
+ *     Timer.run(() { print("executed"); });  // Will never be executed.
  *     foo() {
  *       scheduleMicrotask(foo);  // Schedules [foo] in front of other events.
  *     }
  *     main() {
  *       foo();
  *     }
  *
- * ## Other Resources
+ * ## Other resources
  *
  * * [The Event Loop and Dart](https://www.dartlang.org/articles/event-loop/):
  * Learn how Dart handles the event queue and microtask queue, so you can write
  * better asynchronous code with fewer surprises.
  */
 void scheduleMicrotask(void callback()) {
   if (Zone.current == Zone.ROOT) {
     // No need to bind the callback. We know that the root's scheduleMicrotask
     // will be invoked in the root zone.
     Zone.current.scheduleMicrotask(callback);
     return;
   }
   Zone.current.scheduleMicrotask(
       Zone.current.bindCallback(callback, runGuarded: true));
 }
 
 /**
  * *DEPRECATED*. Use [scheduleMicrotask] instead.
  */
 @deprecated
 void runAsync(void callback()) {
   scheduleMicrotask(callback);
 }
 
 class _AsyncRun {
   /** Schedule the given callback before any other event in the event-loop. */
   external static void _scheduleImmediate(void callback());
 }