Add the 'docRange' property and use it to compute documentation comment.

pkg/analyzer/lib/src/generated/element_handle.dart

 // Copyright (c) 2014, 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.
 
 library engine.element_handle;
 
 import 'ast.dart';
 import 'element.dart';
 import 'engine.dart';
 import 'java_core.dart';
@@ skipping 319 matching lines @@
     return element;
   }
 
   @override
   AnalysisContext get context => _context;
 
   @override
   String get displayName => actualElement.displayName;
 
   @override
+  SourceRange get docRange => actualElement.docRange;
+
+  @override
   Element get enclosingElement => actualElement.enclosingElement;
 
   @override
   int get hashCode => _location.hashCode;
 
   @override
   bool get isDeprecated => actualElement.isDeprecated;
 
   @override
   bool get isOverride => actualElement.isOverride;
@@ skipping 766 matching lines @@
 }
 
 /**
  * TODO(scheglov) invalid implementation
  */
 class WeakReference<T> {
   final T value;
   WeakReference(this.value);
   T get() => value;
 }