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

pkg/analyzer/lib/src/generated/resolver.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;
 
 import 'dart:collection';
 
 import '../task/strong/info.dart' show InferredType, StaticInfo;
 import 'ast.dart';
@@ skipping 4904 matching lines @@
       return;
     }
     // ignore places where the element is not actually used
     if (node.parent is TypeName) {
       if (element is ClassElement) {
         AstNode parent2 = node.parent.parent;
         if (parent2 is IsExpression) {
           return;
         }
         if (parent2 is VariableDeclarationList) {
-          return;
+          // If it's a field's type, it still counts as used.
+          if (parent2.parent is! FieldDeclaration) {
+            return;
+          }
         }
       }
     }
     // OK
     usedElements.addElement(element);
   }
 
   static bool _isReadIdentifier(SimpleIdentifier node) {
     // not reading at all
     if (!node.inGetterContext()) {
@@ skipping 8380 matching lines @@
     nonFields.add(node);
     return null;
   }
 
   @override
   Object visitNode(AstNode node) => node.accept(TypeResolverVisitor_this);
 
   @override
   Object visitWithClause(WithClause node) => null;
 }