anchor.diff:      return '$name$devStr $versionConstraint';
     }
   }
  +
  +VersionConstraint _parseOrNull(String input) {
  +  try {
  +    return new VersionConstraint.parse(input);
  +  } on FormatException catch (e) {
  +    return VersionConstraint.empty;
  +  }
  +}
  
positive.code:   final bool isDevDependency;
  
    Dependency._(this.name, String versionConstraint, this.isDevDependency)
        : this.versionConstraint = new VersionConstraint.parse(versionConstraint);
  
    static Set<Dependency> getDependencies(Map<String, dynamic> yaml) {
      var deps = new Set<Dependency>();
  
negative.code:         if (constraint == null) {
            constraint = '';
          } else if (constraint is Map) {
            constraint = constraint.toString();
          }
  
          var dep = new Dependency._(key, constraint, isDev);
  