address #25232 in strong mode (dart1 behavior unchanged)

pkg/analyzer/lib/src/generated/error_verifier.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.src.generated.error_verifier;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 5025 matching lines @@
           FunctionType foundConcreteFT = _inheritanceManager
               .substituteTypeArgumentsInMemberFromInheritance(
                   concreteType, memberName, enclosingType);
           FunctionType requiredMemberFT = _inheritanceManager
               .substituteTypeArgumentsInMemberFromInheritance(
                   requiredMemberType, memberName, enclosingType);
           foundConcreteFT = _typeSystem.functionTypeToConcreteType(
               _typeProvider, foundConcreteFT);
           requiredMemberFT = _typeSystem.functionTypeToConcreteType(
               _typeProvider, requiredMemberFT);
-          if (_typeSystem.isSubtypeOf(foundConcreteFT, requiredMemberFT)) {
+
+          // Strong mode does override checking for types in CodeChecker, so
+          // we can skip it here. Doing it here leads to unnecessary duplicate
+          // error messages in subclasses that inherit from one that has an
+          // override error.
+          //
+          // See: https://github.com/dart-lang/sdk/issues/25232
+          if (_options.strongMode ||
+              _typeSystem.isSubtypeOf(foundConcreteFT, requiredMemberFT)) {
             continue;
           }
         }
       }
       // The not qualifying concrete executable element was found, add it to the
       // list.
       missingOverrides.add(executableElt);
     }
     // Now that we have the set of missing overrides, generate a warning on this
     // class.
@@ skipping 1838 matching lines @@
 class _InvocationCollector extends RecursiveAstVisitor {
   final List<String> superCalls = <String>[];
 
   @override
   visitMethodInvocation(MethodInvocation node) {
     if (node.target is SuperExpression) {
       superCalls.add(node.methodName.name);
     }
   }
 }