Migrate pub away from throwing strings.

utils/tests/pub/descriptor/tar.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library descriptor.tar;
 
 import 'dart:io';
 import 'dart:async';
 
 import 'package:pathos/path.dart' as path;
@@ skipping 24 matching lines @@
         var file = path.join(parent, name);
         writeBinaryFile(file, bytes);
         return file;
       });
     });
   }, 'creating tar file:\n${describe()}');
 
   /// Validates that the `.tar.gz` file at [path] contains the expected
   /// contents.
   Future validate([String parent]) {
-    throw "TODO(nweiz): implement this";
+    throw new UnimplementedError("TODO(nweiz): implement this");
   }
 
   Stream<List<int>> read() {
     return new Stream<List<int>>.fromFuture(withTempDir((tempDir) {
       return create(tempDir).then((_) =>
           readBinaryFile(path.join(tempDir, name)));
     }));
   }
 }