anchor.diff:  
       var path = versions[i];
   
  -    var versionString =
  -        await HttpRequest.getString("$_storageBase/${path}VERSION");
  +    String versionString;
  +    try {
  +      versionString =
  +          await HttpRequest.getString("$_storageBase/${path}VERSION");
  +    } catch (_) {
  +      continue;
  +    }
   
       var json = JSON.decode(versionString) as Map<String, String>;
   
  
positive.code: 
      // TODO: validate left is "before" right
  
      await _compareVersions(
          _versionMaps[left], _versionMaps[right], includeComments);
      _updateStatus();
    } finally {
      _goButton.disabled = false;
    }
  }
  
  Future _compareVersions(Map left, Map right, bool includeComments) async {
    String leftUri = '$_storageBase/channels/${left['channel']}/release/' +
        '${left['revision']}/api-docs/dart-api-docs.zip';
    String rightUri = '$_storageBase/channels/${right['channel']}/release/' +
        '${right['revision']}/api-docs/dart-api-docs.zip';
  
    var rightData = await getBinaryContent(rightUri);
  
    var leftData = await getBinaryContent(leftUri);
  
    compareZips(
        left, leftData, right, rightData, includeComments, _diffContainer);
  }
  
negative.code:     }
      _goButton.disabled = true;
  
      int left =
          int.parse(_leftVersionSelect.selectedOptions[0].attributes['value']);
      int right =
  