anchor.diff:  
     Map<ExperimentalFlag, bool> computeExperimentalFlags(
         Map<ExperimentalFlag, bool> forcedExperimentalFlags) {
  -    Map<ExperimentalFlag, bool> flags = new Map.from(experimentalFlags);
  +    Map<ExperimentalFlag, bool> flags = new Map.from(defaultExperimentalFlags);
  +    flags.addAll(_experimentalFlags);
       flags.addAll(forcedExperimentalFlags);
       return flags;
     }
  
positive.code:     show CompilerOptions, DiagnosticMessage;
  
  import 'package:front_end/src/api_prototype/experimental_flags.dart'
      show ExperimentalFlag;
  
  import 'package:front_end/src/api_prototype/standard_file_system.dart'
      show StandardFileSystem;
  
negative.code: const String experimentalFlagOptions = '--enable-experiment=';
  
  class TestOptions {
    final Map<ExperimentalFlag, bool> _experimentalFlags;
    final bool forceLateLowering;
    final bool forceNnbdChecks;
    final bool forceNoExplicitGetterCalls;
    final bool nnbdAgnosticMode;
    final String target;
  
    TestOptions(this._experimentalFlags,
        {this.forceLateLowering: false,
        this.forceNnbdChecks: false,
        this.forceNoExplicitGetterCalls: false,
  