futures: handle exceptions immediately if the future already completed.

tests/corelib/src/FutureTest.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 148 matching lines @@
   final future = completer.future;
   final ex = new Exception();
 
   bool reached = false;
   future.handleException((e) { return false; });
   future.handleException((e) { reached = true; return false; }); // overshadowed
   completer.completeException(ex); // future.then is not called, so no exception
   Expect.isTrue(reached);
 }
 
+testExceptionHandlerAfterCompleteThenNotCalled() {
+  final completer = new Completer<int>();
+  final future = completer.future;
+  final ex = new Exception();
+
+  var ex2;
+  completer.completeException(ex);
+  future.handleException((e) { ex2 = e; return true; });
+  future.then((e) { });
+  Expect.equals(ex, ex2);
+}
+
+testExceptionHandlerAfterCompleteReturnsFalseThenThrows() {
+  final completer = new Completer<int>();
+  final future = completer.future;
+  final ex = new Exception();
+
+  var ex2;
+  completer.completeException(ex);
+  future.handleException((e) { ex2 = e; return false; });
+  Expect.throws(() { future.then((e) { }); });
+  Expect.equals(ex, ex2);
+}
+
 // Tests for Future.transform
 
 testTransformSuccess() {
   final completer = new Completer<String>();
   final transformedFuture = completer.future.transform((x) => "** $x **");
   Expect.isFalse(transformedFuture.isComplete);
   completer.complete("42");
   Expect.equals("** 42 **", transformedFuture.value);
 }
 
@@ skipping 77 matching lines @@
   testComplete();
   testCompleteWithHandlerBeforeComplete();
   testCompleteWithHandlerAfterComplete();
   testCompleteManyHandlers();
   testException();
   testExceptionHandler();
   testExceptionHandlerReturnsTrue();
   testExceptionHandlerReturnsTrue2();
   testExceptionHandlerReturnsFalse();
   testExceptionHandlerReturnsFalse2();
+  testExceptionHandlerAfterCompleteThenNotCalled();
+  testExceptionHandlerAfterCompleteReturnsFalseThenThrows();
   testTransformSuccess();
   testTransformFutureFails();
   testTransformTransformerFails();
   testChainSuccess();
   testChainFirstFutureFails();
   testChainTransformerFails();
   testChainSecondFutureFails();
 }