analyzer: Consider "Immutable" annotations the same as "immutable"

Fixes https://github.com/dart-lang/sdk/issues/57670

Change-Id: I5a9ecd153c4e4de21e00860a7743f946096b0849
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/503680
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/lib/src/dart/element/element.dart

@@ -1564,6 +1564,10 @@ class ElementAnnotationImpl
   /// factory.
   static const String _factoryVariableName = 'factory';
 
+  /// The name of the class used to mark a class and its subclasses as being
+  /// immutable.
+  static const String _immutableClassName = 'Immutable';
+
   /// The name of the top-level variable used to mark a class and its subclasses
   /// as being immutable.
   static const String _immutableVariableName = 'immutable';
@@ -1793,7 +1797,9 @@ class ElementAnnotationImpl
   bool get isFactory => _isPackageMetaGetter(_factoryVariableName);
 
   @override
-  bool get isImmutable => _isPackageMetaGetter(_immutableVariableName);
+  bool get isImmutable =>
+      _isPackageMetaGetter(_immutableVariableName) ||
+      _isPackageMetaConstructor(_immutableClassName);
 
   @override
   bool get isInternal => _isPackageMetaGetter(_internalVariableName);
@@ -1949,6 +1955,10 @@ class ElementAnnotationImpl
     return _isTopGetter(libraryName: 'dart.core', name: name);
   }
 
+  bool _isPackageMetaConstructor(String className) {
+    return _isConstructor(libraryName: _metaLibName, className: className);
+  }
+
   bool _isPackageMetaGetter(String name) {
     return _isTopGetter(libraryName: _metaLibName, name: name);
   }

pkg/linter/test/rules/prefer_const_constructors_in_immutables_test.dart

@@ -490,6 +490,19 @@ class C {
     );
   }
 
+  test_immutableInstantiation_nonConstConstructor() async {
+    await assertDiagnostics(
+      r'''
+import 'package:meta/meta.dart';
+@Immutable('')
+class A {
+  A();
+}
+''',
+      [lint(60, 1)],
+    );
+  }
+
   test_implementsImmutable() async {
     await assertNoDiagnostics(r'''
 import 'package:meta/meta.dart';

pkg/linter/test/rules/prefer_const_literals_to_create_immutables_test.dart

@@ -108,6 +108,20 @@ var x = C([]);
     );
   }
 
+  test_listLiteral_noConst_instantiationAnnotation() async {
+    await assertDiagnostics(
+      r'''
+import 'package:meta/meta.dart';
+@Immutable('')
+class C {
+  const C(List<Object> p);
+}
+var x = C([]);
+''',
+      [lint(97, 2)],
+    );
+  }
+
   test_listLiteral_notConstable_noConst() async {
     await assertNoDiagnostics(r'''
 import 'package:meta/meta.dart';