Issue dartdoc#1124. Get used elements from directive comments.

pkg/analyzer/test/generated/hint_code_test.dart

 // Copyright (c) 2016, 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.test.generated.hint_code_test;
 
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/error.dart';
 import 'package:analyzer/src/generated/source_io.dart';
 import 'package:unittest/unittest.dart';
@@ skipping 2691 matching lines @@
         "/lib1.dart",
         r'''
 library lib1;
 class A {}''');
     computeLibrarySourceErrors(source);
     assertErrors(source, [HintCode.UNUSED_IMPORT]);
     assertNoErrors(source2);
     verify([source, source2]);
   }
 
+  void test_unusedImport_inComment_libraryDirective() {
+    Source source = addSource(r'''
+/// Use [Future] class.
+library L;
+import 'dart:async';
+''');
+    assertNoErrors(source);
+  }
+
   void test_unusedImport_show() {
     Source source = addSource(r'''
 library L;
 import 'lib1.dart' show A;
 import 'lib1.dart' show B;
 A a;''');
     Source source2 = addNamedSource(
         "/lib1.dart",
         r'''
 library lib1;
@@ skipping 266 matching lines @@
   n() {
     var a = m(), b = m();
   }
 }''');
     computeLibrarySourceErrors(source);
     assertErrors(
         source, [HintCode.USE_OF_VOID_RESULT, HintCode.USE_OF_VOID_RESULT]);
     verify([source]);
   }
 }