Add code excerpts to new type promotion failures

examples/non_promotion/lib/non_promotion.dart

@@ -1,6 +1,15 @@
 // ignore_for_file: expected_executable, missing_statement
 // ignore_for_file: unused_local_variable, unused_element
 // ignore_for_file: prefer_function_declarations_over_variables
+// ignore_for_file: prefer_if_null_operators
+
+// #docregion not-field, conflicting-getter
+import 'dart:math';
+// #enddocregion not-field, conflicting-getter
+
+// #docregion mock
+import 'package:mockito/mockito.dart';
+// #enddocregion mock
 
 class C1 {
   int? i;
@@ -242,3 +251,137 @@ void miscDeclAnalyzedButNotTested() {
     // #enddocregion closure-write-capture
   }
 }
+
+// #docregion this
+extension on int? {
+  int get valueOrZero {
+    final self = this;
+    return self == null ? 0 : self;
+  }
+}
+// #enddocregion this
+
+// #docregion private
+class PrivateFieldExample {
+  final int? _val;
+  PrivateFieldExample(this._val);
+}
+
+void test(PrivateFieldExample x) {
+  if (x._val != null) {
+    print(x._val + 1);
+  }
+}
+// #enddocregion private
+
+// #docregion final
+class FinalExample {
+  final int? _immutablePrivateField;
+  FinalExample(this._immutablePrivateField);
+
+  void f() {
+    if (_immutablePrivateField != null) {
+      int i = _immutablePrivateField; // OK
+    }
+  }
+}
+// #enddocregion final
+
+// #docregion not-field
+abstract class NotFieldExample {
+  int? get _value => Random().nextBool() ? 123 : null;
+}
+
+void f(NotFieldExample x) {
+  final value = x._value;
+  if (value != null) {
+    print(value.isEven); // OK
+  }
+}
+// #enddocregion not-field
+
+// #docregion external
+class ExternalExample {
+  external final int? _externalField;
+
+  void f() {
+    final i = _externalField;
+    if (i != null) {
+      print(i.isEven); // OK
+    }
+  }
+}
+// #enddocregion external
+
+// #docregion conflicting-getter
+class GetterExample {
+  final int? _overridden;
+  GetterExample(this._overridden);
+}
+
+class Override implements GetterExample {
+  @override
+  int? get _overridden => Random().nextBool() ? 1 : null;
+}
+
+void testParity(GetterExample x) {
+  final i = x._overridden;
+  if (i != null) {
+    print(i.isEven); // OK
+  }
+}
+// #enddocregion conflicting-getter
+
+// #docregion unrelated
+class UnrelatedExample {
+  final int? _i;
+  UnrelatedExample(this._i);
+}
+
+class Unrelated {
+  int? get _j => Random().nextBool() ? 1 : null;
+}
+
+void f2(UnrelatedExample x) {
+  if (x._i != null) {
+    int i = x._i; // OK
+  }
+}
+// #enddocregion unrelated
+
+// #docregion conflicting-field
+class FieldExample {
+  final int? _overridden;
+  FieldExample(this._overridden);
+}
+
+class Override2 implements FieldExample {
+  @override
+  int? _overridden;
+}
+
+void f3(FieldExample x) {
+  final i = x._overridden;
+  if (i != null) {
+    print(i.isEven); // OK
+  }
+}
+// #enddocregion conflicting-field
+
+// #docregion mock
+class MockingExample {
+  final int? _i;
+  MockingExample(this._i);
+}
+
+class MockExample extends Mock implements MockingExample {
+  @override
+  late final int? _i;
+}
+
+void f4(MockingExample x) {
+  if (x._i != null) {
+    int i = x._i; // OK
+  }
+}
+// #enddocregion mock

examples/non_promotion/pubspec.yaml

@@ -5,5 +5,8 @@ version: 0.0.1
 environment:
   sdk: ^3.4.0
 
+dependencies:
+  mockito: ^5.4.4
+
 dev_dependencies:
   lints: ^4.0.0