Migration: add support for Angular's @Optional() annotation.

Bug: https://github.com/dart-lang/sdk/issues/45661
Change-Id: I6c8c87c2a0d26dc8053ef66961a96e03cf5a2582
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/209780
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

pkg/nnbd_migration/lib/instrumentation.dart

@@ -272,6 +272,7 @@ enum EdgeOriginKind {
   nonNullableUsage,
   nonNullAssertion,
   nullabilityComment,
+  optionalAnnotation,
   optionalFormalParameter,
   parameterInheritance,
   quiverCheckNotNull,

pkg/nnbd_migration/lib/src/edge_origin.dart

@@ -542,6 +542,18 @@ class NullabilityCommentOrigin extends EdgeOrigin {
   EdgeOriginKind get kind => EdgeOriginKind.nullabilityComment;
 }
 
+/// Edge origin resulting from the presence of an Angular `@Optional()`
+/// annotation.
+class OptionalAnnotationOrigin extends EdgeOrigin {
+  OptionalAnnotationOrigin(Source? source, AstNode node) : super(source, node);
+
+  @override
+  String get description => "annotated with Angular's @Optional() annotation";
+
+  @override
+  EdgeOriginKind get kind => EdgeOriginKind.optionalAnnotation;
+}
+
 /// Edge origin resulting from the presence of an optional formal parameter.
 ///
 /// For example, in the following code snippet:

pkg/nnbd_migration/lib/src/node_builder.dart

@@ -794,6 +794,15 @@ class NodeBuilder extends GeneralizingAstVisitor<DecoratedType>
       _handleNullabilityHint(node, decoratedType);
     }
     _variables!.recordDecoratedElementType(declaredElement, decoratedType);
+    for (var annotation in node.metadata) {
+      var element = annotation.element;
+      if (element is ConstructorElement &&
+          element.enclosingElement.name == 'Optional' &&
+          _isAngularUri(element.librarySource.uri)) {
+        _graph.makeNullable(
+            decoratedType!.node!, OptionalAnnotationOrigin(source, node));
+      }
+    }
     if (declaredElement.isNamed) {
       _namedParameters![declaredElement.name] = decoratedType;
     } else {
@@ -884,6 +893,18 @@ class NodeBuilder extends GeneralizingAstVisitor<DecoratedType>
         .recordDecoratedDirectSupertypes(declaredElement, decoratedSupertypes);
   }
 
+  /// Determines whether the given [uri] comes from the Angular package.
+  bool _isAngularUri(Uri uri) {
+    if (uri.scheme != 'package') return false;
+    var packageName = uri.pathSegments[0];
+    if (packageName == 'angular') return true;
+    if (packageName == 'third_party.dart_src.angular.angular') {
+      // This name is used for angular development internally at Google.
+      return true;
+    }
+    return false;
+  }
+
   T _pushNullabilityNodeTarget<T>(
       NullabilityNodeTarget target, T Function() fn) {
     NullabilityNodeTarget? previousTarget = _target;

pkg/nnbd_migration/test/abstract_context.dart

@@ -45,6 +45,27 @@ class AbstractContextTest with ResourceProviderMixin {
 
   String get testsPath => '$homePath/tests';
 
+  /// Makes a mock version of the Angular package available for unit testing.
+  ///
+  /// If optional argument [internalUris] is `true`, the mock Angular package
+  /// will be located in a package called `third_party.dart_src.angular.angular`
+  /// (as it is in Google3), and `package:angular` will simply re-export it;
+  /// this allows the test to reflect usage in internal sources.
+  void addAngularPackage({bool internalUris = false}) {
+    addPackageFile(
+        internalUris ? 'third_party.dart_src.angular.angular' : 'angular',
+        'angular.dart', '''
+class Optional {
+  const Optional();
+}
+''');
+    if (internalUris) {
+      addPackageFile('angular', 'angular.dart', '''
+export 'package:third_party.dart_src.angular.angular/angular.dart';
+''');
+    }
+  }
+
   void addBuiltValuePackage() {
     addPackageFile('built_value', 'built_value.dart', '''
 abstract class Built<V extends Built<V, B>, B extends Builder<V, B>> {}

pkg/nnbd_migration/test/api_test.dart

@@ -356,6 +356,65 @@ g() {
     await _checkSingleFileChanges(content, expected);
   }
 
+  Future<void> test_angular_optional_constructor_param() async {
+    addAngularPackage();
+    var content = '''
+import 'package:angular/angular.dart';
+
+class MyComponent {
+  MyComponent(@Optional() String foo);
+}
+''';
+    var expected = '''
+import 'package:angular/angular.dart';
+
+class MyComponent {
+  MyComponent(@Optional() String? foo);
+}
+''';
+    await _checkSingleFileChanges(content, expected);
+  }
+
+  Future<void> test_angular_optional_constructor_param_field_formal() async {
+    addAngularPackage();
+    var content = '''
+import 'package:angular/angular.dart';
+
+class MyComponent {
+  String foo;
+  MyComponent(@Optional() this.foo);
+}
+''';
+    var expected = '''
+import 'package:angular/angular.dart';
+
+class MyComponent {
+  String? foo;
+  MyComponent(@Optional() this.foo);
+}
+''';
+    await _checkSingleFileChanges(content, expected);
+  }
+
+  Future<void> test_angular_optional_constructor_param_internal() async {
+    addAngularPackage(internalUris: true);
+    var content = '''
+import 'package:angular/angular.dart';
+
+class MyComponent {
+  MyComponent(@Optional() String foo);
+}
+''';
+    var expected = '''
+import 'package:angular/angular.dart';
+
+class MyComponent {
+  MyComponent(@Optional() String? foo);
+}
+''';
+    await _checkSingleFileChanges(content, expected);
+  }
+
   Future<void> test_argumentError_checkNotNull_implies_non_null_intent() async {
     var content = '''
 void f(int i) {