Add test for canonicalization edge case

Author: jcollins-g

test/model_test.dart

@@ -1389,6 +1389,17 @@ void main() {
   });
 
   group('Class edge cases', () {
+    test('Inherit from private class across private library to public library',
+        () {
+      Class GadgetExtender = packageGraph.localPublicLibraries
+          .firstWhere((l) => l.name == 'gadget_extender')
+          .allClasses
+          .firstWhere((c) => c.name == 'GadgetExtender');
+      Field gadgetGetter =
+          GadgetExtender.allFields.firstWhere((f) => f.name == 'gadgetGetter');
+      expect(gadgetGetter.isCanonical, isTrue);
+    });
+
     test(
         'ExecutableElements from private classes and from public interfaces (#1561)',
         () {

test/src/utils.dart

@@ -17,7 +17,7 @@ import 'package:path/path.dart' as path;
 /// The number of public libraries in testing/test_package, minus 2 for
 /// the excluded libraries listed in the initializers for _testPackageGraphMemo
 /// and minus 1 for the <nodoc> tag in the 'excluded' library.
-const int kTestPackagePublicLibraries = 15;
+const int kTestPackagePublicLibraries = 16;
 
 final RegExp quotables = RegExp(r'[ "\r\n\$]');
 final RegExp observatoryPortRegexp =

testing/test_package/lib/gadget_extender.dart

@@ -0,0 +1,7 @@
+// Copyright (c) 2020, 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.
+
+import 'src/gadget.dart';
+
+class GadgetExtender extends Gadget {}

testing/test_package/lib/src/gadget.dart

@@ -0,0 +1,10 @@
+// Copyright (c) 2020, 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.
+
+class _GadgetBase {
+  int get gadgetGetter => 5;
+}
+
+
+class Gadget extends _GadgetBase {}