anchor.diff:    }
     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;
  +}
  
positive.code: // BSD-style license that can be found in the LICENSE file.
  
  import 'dart:io' as io;
  
  import 'package:cli_util/cli_util.dart' as cli_util;
  import 'package:services/src/analysis_server.dart' as analysis_server;
  import 'package:services/src/analyzer.dart' as ana;
  import 'package:services/src/compiler.dart' as comp;
  
  main (List<String> args) async {
    if (args.length == 0) {
      print ("Usage: slow_test path_to_test_collection");
      io.exit(1);
    }
  
    String testCollectionRoot = args[0];
  
    io.Directory sdkDir = cli_util.getSdkDir([]);
    var analysisServer = new analysis_server.AnalysisServerWrapper(sdkDir.path);
  
negative.code: 
    var f = new io.File(path);
    String src = f.readAsStringSync();
    f = null;
  
    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 {
  