anchor.diff:  
     var f = new io.File(path);
     String src = f.readAsStringSync();
  -  var averageCompletionTime = await testCompletions(src, wrapper);
  -  var averageAnalysisTime = await testAnalysis(src, analyzer);
  -  var averageDocumentTime = await testDocument(src, analyzer);
  -  var averageCompilationTime = await testCompilation(src, compiler);
  -  var averageFixesTime = await testFixes(src, wrapper);
  +  f = null;
   
  -  print (
  -      "$path, "
  +  for (int i = 0; i < 5; i++) {
  +    int noChanges = r.nextInt(20);
  +
  +    for (int j = 0; j < noChanges; j++) {
  +      src = mutate(src);
  +    }
  +
  +    var averageCompletionTime = await testCompletions(src, wrapper);
  +    var averageAnalysisTime = await testAnalysis(src, analyzer);
  +    var averageDocumentTime = await testDocument(src, analyzer);
  +    var averageCompilationTime = await testCompilation(src, compiler);
  +    var averageFixesTime = await testFixes(src, wrapper);
  +
  +    print (
  +      "$path-$i - $noChanges, "
         "${averageCompilationTime.toStringAsFixed(2)}ms, "
         "${averageAnalysisTime.toStringAsFixed(2)}ms, "
         "${averageCompletionTime.toStringAsFixed(2)}ms, "
         "${averageDocumentTime.toStringAsFixed(2)}ms, "
         "${averageFixesTime.toStringAsFixed(2)}ms"
         );
  +  }
   }
   
   testAnalysis(String src, ana.Analyzer analyzer) async {
  
positive.code:     if (!fse.path.endsWith('.dart')) continue;
  
      try {
        await testPath(fse.path, analysisServer, analyzer, compiler);
      } catch (e) {
        print (e);
  
negative.code:   }
    return sw.elapsedMilliseconds / src.length;
  }
  
  mutate(String src) {
    var c = ["{", "}", "[", "]", "'", ",", "!", "@", "#", "\$", "%",
      "^", "&", " ", "(", ")", "null ", "class ", "for ", "void ", "var ",
      "dynamic ", ";", "as ", "is ", ".", "import "];
    String s = c[r.nextInt(c.length)];
    int i = r.nextInt(src.length);
    if (i == 0) i = 1;
    String newStr = src.substring(0, i - 1) + s + src.substring(i);
    return newStr;
  }
  