anchor.diff:  import 'package:front_end/src/fasta/type_inference/type_schema_environment.dart';
   import 'package:kernel/ast.dart';
   import 'package:kernel/class_hierarchy.dart';
  -import 'package:kernel/text/ast_to_text.dart';
   import 'package:kernel/transformations/flags.dart' show TransformerFlag;
   import 'package:kernel/type_algebra.dart';
   import 'package:kernel/type_environment.dart';
  -
  -/// Set this flag to `true` to cause debugging information about covariance
  -/// checks to be printed to standard output.
  -const bool debugCovariance = false;
  -
  -/// Type of a closure which applies a covariance annotation to a class member.
  -///
  -/// This is necessary since we need to determine which covariance annotations
  -/// need to be added before creating a forwarding stub, but the covariance
  -/// annotations themselves need to be applied to the forwarding stub.
  -typedef void _CovarianceFix(FunctionNode function);
   
   /// Concrete class derived from [InferenceNode] to represent type inference of
   /// getters, setters, and fields based on inheritance.
  
positive.code:           isGenericCovariantImpl = true;
          }
        }
        if (isGenericCovariantImpl != superTypeParameter.isGenericCovariantImpl) {
          createImplIfNeeded();
        }
        if (isGenericCovariantImpl != typeParameter.isGenericCovariantImpl) {
          fixes.add((FunctionNode function) => function
              .typeParameters[i].isGenericCovariantImpl = isGenericCovariantImpl);
        }
      }
  
      if (debugCovariance && fixes.isNotEmpty) {
        print('  ${fixes.length} fix(es)');
      }
    }
  
    void _createForwardingImplIfNeeded(FunctionNode function) {
  
negative.code:           isCovariant = true;
          }
        }
        if (isGenericCovariantImpl) {
          if (!superParameter.isGenericCovariantImpl) {
            createImplIfNeeded();
          }
          if (!parameter.isGenericCovariantImpl) {
            createStubIfNeeded();
            stub.function.positionalParameters[i].isGenericCovariantImpl = true;
          }
        }
        if (isCovariant) {
          if (!superParameter.isCovariant) {
            createImplIfNeeded();
          }
          if (!parameter.isCovariant) {
            createStubIfNeeded();
            stub.function.positionalParameters[i].isCovariant = true;
          }
        }
      }
      for (int i = 0; i < interfaceNamedParameters.length; i++) {
  