[linter] Suppress prefer_initializing_formals when not a subtype.

Fixes the `prefer_initializing_formals` lint so that it no longer
fires on code like this:

    class C {
      int? _x;

      C({dynamic x}) : _x = x;
    }

The reason is that the type of an initializing formal parameter is
required to be a subtype of the type of the field; coercions and
dynamic downcasts are not allowed. So in the example above, there is
no semantics-preserving way to convert `x` to an initializing formal.

Note that this situation occasionally crops up due to the parameter
having an implicit type, e.g.:

    class C {
      int? _x;

      C({x}) : _x = x;
    }

Fixes https://github.com/dart-lang/sdk/issues/62765.

Change-Id: I6a6a69648d3b2bd2f631ef01093b199125a71fa7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/483864
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

pkg/linter/lib/src/rules/prefer_initializing_formals.dart

@@ -142,6 +142,15 @@ class _ConstructorChecker {
     if (parameter is! FormalParameterElement) return;
     if (!_parameters.contains(parameter)) return;
 
+    // An initializing formal is required to have a type that's a subtype of the
+    // field type (assignability is not sufficient). If this requirement isn't
+    // met, don't lint, because the corresponding fix will lead to a
+    // compile-time error.
+    var library = parameter.library!;
+    if (!library.typeSystem.isSubtypeOf(parameter.type, field.type)) {
+      return;
+    }
+
     // Must be the same name (modulo privacy for private named parameters).
     if (field.isPrivate) {
       // Never lint on private names if the feature isn't supported.

pkg/linter/test/rules/prefer_initializing_formals_test.dart

@@ -396,6 +396,42 @@ class C {
 ''');
   }
 
+  test_dynamicParameterType_dynamicField() async {
+    await assertDiagnostics(
+      r'''
+class C {
+  dynamic _x;
+
+  C({dynamic x}) : _x = x;
+}
+''',
+      [lint(44, 6)],
+    );
+  }
+
+  test_dynamicParameterType_nonTopTypeField() async {
+    await assertNoDiagnostics(r'''
+class C {
+  String? _x;
+
+  C({dynamic x}) : _x = x;
+}
+''');
+  }
+
+  test_dynamicParameterType_objectQuestionField() async {
+    await assertDiagnostics(
+      r'''
+class C {
+  Object? _x;
+
+  C({dynamic x}) : _x = x;
+}
+''',
+      [lint(44, 6)],
+    );
+  }
+
   test_factoryConstructor() async {
     // https://github.com/dart-lang/linter/issues/2441
     await assertNoDiagnostics(r'''
@@ -429,6 +465,42 @@ class C {
 ''');
   }
 
+  test_implicitParameterType_dynamicField() async {
+    await assertDiagnostics(
+      r'''
+class C {
+  dynamic _x;
+
+  C({x}) : _x = x;
+}
+''',
+      [lint(36, 6)],
+    );
+  }
+
+  test_implicitParameterType_nonTopTypeField() async {
+    await assertNoDiagnostics(r'''
+class C {
+  String? _x;
+
+  C({x}) : _x = x;
+}
+''');
+  }
+
+  test_implicitParameterType_objectQuestionField() async {
+    await assertDiagnostics(
+      r'''
+class C {
+  Object? _x;
+
+  C({x}) : _x = x;
+}
+''',
+      [lint(36, 6)],
+    );
+  }
+
   test_initializeFromOtherParameter() async {
     await assertNoDiagnostics(r'''
 class C {