anchor.diff:    final int actualPrefixLength = 'Actual: '.length;
     TestResult currentResult;
     Map<String, List<TestResult>> testsByExpectedAndActual =
  +      <String, List<TestResult>>{};
  +  Map<String, List<TestResult>> testsByStackTrace =
         <String, List<TestResult>>{};
     while (index < output.length) {
       String currentLine = output[index];
  
positive.code:   List<String> stackTrace;
  
    TestResult(this.testName, this.expected, this.actual);
  }
  
negative.code:         }
          if (hasStackTrace) {
            currentResult.stackTrace = output.sublist(index + 1, endIndex - 2);
            String traceLine = currentResult.traceLine;
            testsByStackTrace
                .putIfAbsent(traceLine, () => <TestResult>[])
                .add(currentResult);
          }
          index = endIndex;
        }
  