set LibraryElement documentation to docs of first directive in CU

pkg/analyzer/lib/src/task/dart.dart

 // Copyright (c) 2015, 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 analyzer.src.task.dart;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
@@ skipping 1474 matching lines @@
     }
     LibraryElementImpl libraryElement =
         new LibraryElementImpl.forNode(owningContext, libraryNameNode);
     libraryElement.definingCompilationUnit = definingCompilationUnitElement;
     libraryElement.entryPoint = entryPoint;
     libraryElement.parts = sourcedCompilationUnits;
     for (Directive directive in directivesToResolve) {
       directive.element = libraryElement;
     }
     BuildLibraryElementUtils.patchTopLevelAccessors(libraryElement);
-    if (libraryDirective != null) {
-      _setDoc(libraryElement, libraryDirective);
+    // set the library documentation to the docs associated with the first
+    // directive in the compilation unit.
+    if (definingCompilationUnit.directives.isNotEmpty) {
+      _setDoc(libraryElement, definingCompilationUnit.directives.first);

[Brian Wilkerson, 2016/01/22 22:44:52]

Given that this was the only use of `libraryDirective`, it would be good to
remove it.

[keertip, 2016/01/23 00:12:04]

Done.

     }
+
     //
     // Record outputs.
     //
     outputs[BUILD_LIBRARY_ERRORS] = errors;
     outputs[LIBRARY_ELEMENT1] = libraryElement;
     outputs[IS_LAUNCHABLE] = entryPoint != null;
   }
 
   /**
    * Return the top-level [FunctionElement] entry point, or `null` if the given
@@ skipping 3941 matching lines @@
 
   @override
   bool moveNext() {
     if (_newSources.isEmpty) {
       return false;
     }
     currentTarget = _newSources.removeLast();
     return true;
   }
 }