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

pkg/analyzer/test/generated/resolver_test.dart

Index: pkg/analyzer/test/generated/resolver_test.dart
diff --git a/pkg/analyzer/test/generated/resolver_test.dart b/pkg/analyzer/test/generated/resolver_test.dart
index ecbbb2b99e5724618e4e4fff23a9c702bf92afa7..7d199e0d85a7495862a7b8703ce8c1ad236d8a3c 100644
--- a/pkg/analyzer/test/generated/resolver_test.dart
+++ b/pkg/analyzer/test/generated/resolver_test.dart
@@ -3762,6 +3762,22 @@ main() {
     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'''