analyzer: Change the mock SDK Enum class to declare index and _name as concrete

This allows us to write enums in the mock SDK (which is currently broken). This will allow me to land such an enum in the SDK, in
https://dart-review.googlesource.com/c/sdk/+/445403.

This diverges from the actual Dart SDK implementation of the Enum class
and the _Enum class. I think this is an acceptable divergence; these
classes hardly ever change, and the changes will be easy to follow
when they come, however many months or years from now.

Change-Id: Ia6ff3803415af8c251dbee330a218f2618395cc4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/446141
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>

Author: srawlins

pkg/analysis_server/test/analysis/notification_navigation_test.dart

@@ -941,7 +941,7 @@ void f() {
 ''');
     await prepareNavigation();
     assertHasRegion('index');
-    assertHasTargetInDartCore('index; // Enum');
+    assertHasTargetInDartCore('index => -1; // Enum');
   }
 
   Future<void> test_enum_method() async {

pkg/analyzer/lib/src/test_utilities/mock_sdk.dart

@@ -406,8 +406,14 @@ class Duration implements Comparable<Duration> {
 
 @Since("2.14")
 abstract class Enum {
-  int get index; // Enum
-  String get _name;
+  // These two getters are a departure here, from the real SDK. Here we have
+  // concrete getters; the SDK has abstract gettesr. The real implementations
+  // of enums require a significant amount of desugaring.
+  // (See the spec:
+  // https://github.com/dart-lang/language/blob/main/accepted/2.17/enhanced-enums/feature-specification.md#implementation).
+  // As an alternative to desugaring, we make these getters concrete.
+  int get index => -1; // Enum; this comment is necessary for notification_navigation_test.
+  String get _name => '';
 }
 
 abstract class _Enum implements Enum {