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

tests/corelib/future_test.dart

 // Copyright (c) 2012, 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.
 
 // Tests for Future.immediate
 
 testImmediate() {
   final future = new Future<String>.immediate("42");
   Expect.isTrue(future.isComplete);
   Expect.isTrue(future.hasValue);
@@ skipping 526 matching lines @@
     called = true;
     return "transformed value";
   });
 
   completer.completeException("original error");
   Expect.isTrue(called);
   Expect.isTrue(transformedFuture.isComplete);
   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();
   testComplete();
   testCompleteWithCompleteHandlerBeforeComplete();
   testExceptionWithCompleteHandlerBeforeComplete();
   testCompleteWithCompleteHandlerAfterComplete();
   testExceptionWithCompleteHandlerAfterComplete();
   testCompleteWithManyCompleteHandlers();
   testExceptionWithManyCompleteHandlers();
@@ skipping 19 matching lines @@
   testTransformSuccess();
   testTransformFutureFails();
   testTransformTransformerFails();
   testChainSuccess();
   testChainFirstFutureFails();
   testChainTransformerFails();
   testChainSecondFutureFails();
   testTransformExceptionCompletesNormally();
   testTransformExceptionThrows();
   testTransformExceptionReturns();
+  testTransformExceptionReturnsAFuture();
 }