A class that's only used as a field type should be considered used

pkg/analyzer/test/generated/resolver_test.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.resolver_test;
 
 import 'dart:collection';
 
 import 'package:analyzer/src/context/context.dart';
 import 'package:analyzer/src/generated/ast.dart';
@@ skipping 3744 matching lines @@
     enableUnusedElement = true;
     Source source = addSource(r'''
 class _A {}
 main() {
 }''');
     computeLibrarySourceErrors(source);
     assertErrors(source, [HintCode.UNUSED_ELEMENT]);
     verify([source]);
   }
 
+  void test_unusedElement_class_isUsed_fieldDeclaration() {
+    enableUnusedElement = true;
+    var src = r'''
+class Foo {
+  _Bar x;
+}
+
+class _Bar {
+}
+''';
+    Source source = addSource(src);
+    computeLibrarySourceErrors(source);
+    assertNoErrors(source);
+    verify([source]);
+  }
+
   void test_unusedElement_class_notUsed_variableDeclaration() {
     enableUnusedElement = true;
     Source source = addSource(r'''
 class _A {}
 main() {
   _A v;
   print(v);
 }
 print(x) {}
 ''');
@@ skipping 12274 matching lines @@
 
   void _resolveTestUnit(String code) {
     testCode = code;
     testSource = addSource(testCode);
     LibraryElement library = resolve2(testSource);
     assertNoErrors(testSource);
     verify([testSource]);
     testUnit = resolveCompilationUnit(testSource, library);
   }
 }