If transformException() returns a future, act like chain().

tests/corelib/future_test.dart

Index: tests/corelib/future_test.dart
diff --git a/tests/corelib/future_test.dart b/tests/corelib/future_test.dart
index aefe3ff99abccba00e29aa4e9e19d0a774c426de..4544a18b19774b10f9fa7f76ccd004b36c2219d9 100644
--- a/tests/corelib/future_test.dart
+++ b/tests/corelib/future_test.dart
@@ -544,6 +544,27 @@ testTransformExceptionReturns() {
   Expect.equals("transformed value", transformedFuture.value);
 }
 
+testTransformExceptionReturnsAFuture() {
+  final completer = new Completer<String>();
+  var called = false;
+
+  final returnedCompleter = new Completer<String>();
+
+  final transformedFuture = completer.future.transformException((ex) {
+    Expect.equals("original error", ex);
+    called = true;
+    return returnedCompleter.future;
+  });
+
+  completer.completeException("original error");
+  Expect.isTrue(called);
+  Expect.isFalse(transformedFuture.isComplete);
+
+  returnedCompleter.complete("transformed value");
+  Expect.isTrue(transformedFuture.isComplete);
+  Expect.equals("transformed value", transformedFuture.value);
+}
+
 main() {
   testImmediate();
   testNeverComplete();
@@ -583,4 +604,5 @@ main() {
   testTransformExceptionCompletesNormally();
   testTransformExceptionThrows();
   testTransformExceptionReturns();
+  testTransformExceptionReturnsAFuture();
 }