Update headings to use sentence case.

sdk/lib/js/dart2js/js_dart2js.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.
 
 /**
  * Support for interoperating with JavaScript.
  * 
  * This library provides access to JavaScript objects from Dart, allowing
  * Dart code to get and set properties, and call methods of JavaScript objects
  * and invoke JavaScript functions. The library takes care of converting
@@ skipping 25 matching lines @@
  *     import 'dart:js';
  *     
  *     main() {
  *       var object = new JsObject(context['Object']);
  *       object['greeting'] = 'Hello';
  *       object['greet'] = (name) => "${object['greeting']} $name";
  *       var message = object.callMethod('greet', ['JavaScript']);
  *       context['console'].callMethod('log', [message]);
  *     }
  *
- * ## Proxying and Automatic Conversion
+ * ## Proxying and automatic conversion
  * 
  * When setting properties on a JsObject or passing arguments to a Javascript
  * method or function, Dart objects are automatically converted or proxied to
  * JavaScript objects. When accessing JavaScript properties, or when a Dart
  * closure is invoked from JavaScript, the JavaScript objects are also
  * converted to Dart.
  *
  * Functions and closures are proxied in such a way that they are callable. A
  * Dart closure assigned to a JavaScript property is proxied by a function in
  * JavaScript. A JavaScript function accessed from Dart is proxied by a
@@ skipping 471 matching lines @@
 }
 
 Object _getDartProxy(o, String propertyName, createProxy(o)) {
   var dartProxy = JS('', '#[#]', o, propertyName);
   if (dartProxy == null) {
     dartProxy = createProxy(o);
     _defineProperty(o, propertyName, dartProxy);
   }
   return dartProxy;
 }