anchor.diff:  }
   
   void performErrorAndWarningCheck(
  -    Map<String, dynamic> world,
  +    YamlMap world,
       bool gotError,
       List<String> formattedErrors,
       bool gotWarning,
  
positive.code:     if (brandNewWorld) {
        sourceFiles = new Map<String, String>.from(world["sources"]);
      } else {
        sourceFiles.addAll(world["sources"]);
      }
      Uri packagesUri;
      for (String filename in sourceFiles.keys) {
  
negative.code:   Map<String, String> sourceFiles;
    CompilerOptions options;
    TestIncrementalCompiler compiler;
    for (YamlMap world in worlds) {
      bool brandNewWorld = true;
      if (world["worldType"] == "updated") {
        brandNewWorld = false;
  