Move a test case from e2e tests to unit tests.

This test case is tickled in a refactoring I am attempting. I'm moving it to be
a unit test here, for better debuggability, and understandability, before I
make the refactoring.

Author: srawlins

test/classes_test.dart

@@ -197,5 +197,37 @@ class D implements Object {}
     expect(toString.canonicalEnclosingContainer!.isDartCoreObject, isTrue);
   }
 
+  void test_multiplyInheritedOperator_oneIsPrivate() async {
+    // Test an edge case where inherited ExecutableElements can come both from
+    // private classes and public interfaces.  The test makes sure the class
+    // still takes precedence.
+    // See https://github.com/dart-lang/dartdoc/issues/1561.
+    var library = await bootPackageWithLibrary('''
+abstract class A<K, V> {
+  void operator []=(K key, V value);
+}
+
+abstract class B<K, V> implements A<K, V> {
+  @override
+  operator []=(K key, V value);
+}
+
+abstract class C<K, V> extends B<K, V> {}
+
+abstract class _D<K, V> implements A<K, V> {
+  @override
+  void operator []=(K key, V value);
+}
+
+abstract class E<K, V> = C<K, V> with _D<K, V>;
+''');
+
+    var indexAssign =
+        library.classes.named('E').inheritedOperators.named('operator []=');
+    expect(indexAssign.element2.enclosingElement2!.name3, '_D');
+    expect(indexAssign.canonicalEnclosingContainer!.name, 'E');
+    expect(indexAssign.canonicalModelElement!.enclosingElement!.name, 'E');
+  }
+
   // TODO(srawlins): Test everything else about classes.
 }

test/end2end/model_test.dart

@@ -4,8 +4,6 @@
 
 // ignore_for_file: non_constant_identifier_names
 
- // ignore_for_file: analyzer_use_new_elements
-
 import 'package:analyzer/dart/element/element2.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/source/line_info.dart';
@@ -909,7 +907,7 @@ void main() async {
     });
 
     test('can import other libraries with unusual URIs', () {
-        final importLists = fakeLibrary.element2.fragments
+      final importLists = fakeLibrary.element2.fragments
           .map((fragment) => fragment.libraryImports2);
       final exportLists = fakeLibrary.element2.fragments
           .map((fragment) => fragment.libraryExports2);
@@ -1680,19 +1678,6 @@ void main() async {
       var gadgetGetter = GadgetExtender.instanceFields.named('gadgetGetter');
       expect(gadgetGetter.isCanonical, isTrue);
     });
-
-    test(
-        'ExecutableElements from private classes and from public interfaces (#1561)',
-        () {
-      var MIEEMixinWithOverride =
-          fakeLibrary.classes.wherePublic.named('MIEEMixinWithOverride');
-      var problematicOperator =
-          MIEEMixinWithOverride.inheritedOperators.named('operator []=');
-      expect(problematicOperator.element2.enclosingElement2?.name3,
-          equals('_MIEEPrivateOverride'));
-      expect(problematicOperator.canonicalModelElement!.enclosingElement!.name,
-          equals('MIEEMixinWithOverride'));
-    });
   });
 
   group('Mixin', () {

testing/test_package/lib/fake.dart

@@ -1117,34 +1117,6 @@ extension OnOldSchool on OldSchoolMixin {
 
 class School with OldSchoolMixin, NewSchoolMixin {}
 
-//
-//
-//
-
-/// Test an edge case for cases where inherited ExecutableElements can come
-/// both from private classes and public interfaces.  The test makes sure the
-/// class still takes precedence (#1561).
-abstract class MIEEMixinWithOverride<K, V> = MIEEBase<K, V>
-    with _MIEEPrivateOverride<K, V>;
-
-abstract class _MIEEPrivateOverride<K, V> implements MIEEThing<K, V> {
-  // ignore: annotate_overrides
-  void operator []=(K key, V value) {
-    throw UnsupportedError("Never use this");
-  }
-}
-
-abstract class MIEEBase<K, V> extends MIEEMixin<K, V> {}
-
-abstract class MIEEMixin<K, V> implements MIEEThing<K, V> {
-  // ignore: annotate_overrides
-  operator []=(K key, V value);
-}
-
-abstract class MIEEThing<K, V> {
-  void operator []=(K key, V value);
-}
-
 abstract class ImplementingClassForTool {
   /// Invokes a tool from inherited documentation via `implemented`
   ///