Redo StreamTransformers so they work with Stack traces.

pkg/barback/test/stream_pool_test.dart

Index: pkg/barback/test/stream_pool_test.dart
diff --git a/pkg/barback/test/stream_pool_test.dart b/pkg/barback/test/stream_pool_test.dart
index d7013292cb42551de8a5467f5c47c6bfab2dfba5..fafa1f502cb52ffa4680f5256145b28a70037a40 100644
--- a/pkg/barback/test/stream_pool_test.dart
+++ b/pkg/barback/test/stream_pool_test.dart
@@ -50,9 +50,11 @@ main() {
       controller1.add("fifth");
 
       expect(newFuture(() {
-        return pool.stream.transform(new StreamTransformer(
+        return pool.stream.transform(new StreamTransformer.fromHandlers(
             handleData: (data, sink) => sink.add(["data", data]),
-            handleError: (error, sink) => sink.add(["error", error]))).toList();
+            handleError: (error, stackTrace, sink) {
+          sink.add(["error", error]);
+        })).toList();
       }), completion(equals([
         ["data", "first"],
         ["data", "second"],
@@ -111,9 +113,10 @@ main() {
       controller2.addError("second");
 
       expect(newFuture(() {
-        return pool.stream.transform(new StreamTransformer(
+        return pool.stream.transform(new StreamTransformer.fromHandlers(
             handleData: (data, sink) => sink.add(data),
-            handleError: (error, sink) => sink.add(error))).toList();
+            handleError: (error, stackTrace, sink) { sink.add(error); }))
+            .toList();
       }), completion(isEmpty));
 
       pumpEventQueue().then((_) => pool.close());