anchor.diff:        for (var fix in covarianceFixes) {
           fix(function);
         }
  -      if (inheritedMember is SyntheticAccessor) {
  -        var field = inheritedMember._field;
  -        if (inheritedMember.kind == ProcedureKind.Setter) {
  -          // Propagate covariance fixes to the field.
  -          var setterParameter = function.positionalParameters[0];
  -          field.isCovariant = setterParameter.isCovariant;
  -          field.isGenericCovariantInterface =
  -              setterParameter.isGenericCovariantInterface;
  -          field.isGenericCovariantImpl = setterParameter.isGenericCovariantImpl;
  -        }
  -      }
         return inheritedMember;
       }
     }
  
positive.code: 
    SyntheticAccessor(
        Name name, ProcedureKind kind, FunctionNode function, this._field)
        : super(name, kind, function);
  
    @override
    int get fileOffset => _field.fileOffset;
  
    @override
    String get fileUri => _field.fileUri;
  
    static getField(SyntheticAccessor accessor) => accessor._field;
  }
  
negative.code:     var declaredTypeParameters = _declaredMethod.function.typeParameters;
      for (int i = _start; i < _end; i++) {
        var candidate = _candidates[i];
        if (candidate is SyntheticAccessor) {
          // This can happen if there are errors.  Just skip this override.
          continue;
        }
        var candidateFunction = candidate.function;
        if (candidateFunction == null) {
          // This can happen if there are errors.  Just skip this override.
  