anchor.diff:    cancel() {}
   }
   """;
  +
  +/// A minimal implementation of dart:collection that is sufficient to create an
  +/// instance of [CoreTypes] and compile program.
  +const String defaultDartCollectionSource = """
  +class _UnmodifiableSet {
  +  final Map _map;
  +  const _UnmodifiableSet(this._map);
  +}
  +""";
  +
  +/// A minimal implementation of dart:_internel that is sufficient to create an
  +/// instance of [CoreTypes] and compile program.
  +const String defaultDartInternalSource = """
  +class Symbol {
  +  const Symbol(String name);
  +}
  +""";
  
positive.code: 
  class Symbol {}
  
  class Type {}
  
  class _InvocationMirror {
  
negative.code: 
        case "dart:async":
          return utf8.encode(defaultDartAsyncSource);
  
        case "dart:collection":
          return utf8.encode(defaultDartCollectionSource);
  
        case "dart:_internal":
          return utf8.encode(defaultDartInternalSource);
  
        default:
          return utf8.encode(message == null ? "" : "/* ${message.message} */");
  