Make sure to drain stderr in tests so that spawned process doesn't hang.

utils/pub/io.dart

Index: utils/pub/io.dart
diff --git a/utils/pub/io.dart b/utils/pub/io.dart
index 968f3546fbc4bcf77b02797f0ab46dc99357435c..1a5a7b330fe679ccf9c63534a1d15ce7519bb52d 100644
--- a/utils/pub/io.dart
+++ b/utils/pub/io.dart
@@ -605,6 +605,19 @@ Future<String> consumeStringInputStream(StringInputStream stream) {
   return completer.future;
 }
 
+/// Wrap an InputStream in a ListInputStream. This eagerly drains the [source]
+/// input stream. This is useful for spawned processes which will not exit until
+/// their output streams have been drained.
+/// TODO(rnystrom): Get rid of this once #7218 is fixed.
+InputStream wrapInputStream(InputStream source) {
+  var sink = new ListInputStream();
+  // TODO(nweiz): Due to issuee 3657, pipeInputToInput naturally avoids calling
+  // both onClosed and onError. If 3657 gets fixed before 7013, we'll need to do
+  // that explicitly.
+  pipeInputToInput(source, sink);
+  return sink;
+}
+
 /// Spawns and runs the process located at [executable], passing in [args].
 /// Returns a [Future] that will complete with the results of the process after
 /// it has ended.