Show nicer errors on some server failures.

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 424 matching lines @@
 testTransformTransformerFails() {
   final completer = new Completer<String>();
   final error = new Exception("Oh no!");
   final transformedFuture = completer.future.transform((x) { throw error; });
   Expect.isFalse(transformedFuture.isComplete);
   transformedFuture.then((v) => null);
   Expect.throws(() => completer.complete("42"), check: (e) => e == error);
   Expect.equals(error, transformedFuture.exception);
 }
 
+testTransformOnExceptionTransformerThrows() {
+  final completer = new Completer<String>();
+  var called = false;
+
+  final transformedFuture = completer.future.transform((x) {
+    throw "throw from transform";
+  }, (ex) {
+    Expect.equals("throw from transform", ex);
+    called = true;
+    throw "transformed error";
+  });
+
+  completer.complete("original error");
+  Expect.isTrue(called);
+  Expect.isTrue(transformedFuture.isComplete);
+  Expect.equals("transformed error", transformedFuture.exception);
+}
+
+testTransformOnExceptionThrows() {
+  final completer = new Completer<String>();
+  var called = false;
+
+  final transformedFuture = completer.future.transform((x) {
+    Expect.fail("should not get here");
+  }, (ex) {
+    Expect.equals("original error", ex);
+    called = true;
+    throw "transformed error";
+  });
+
+  completer.completeException("original error");
+  Expect.isTrue(called);
+  Expect.isTrue(transformedFuture.isComplete);
+  Expect.equals("transformed error", transformedFuture.exception);
+}
+
+testTransformOnExceptionReturnsTrue() {
+  final completer = new Completer<String>();
+  var called = false;
+
+  final transformedFuture = completer.future.transform((x) {
+    Expect.fail("should not get here");
+  }, (ex) {
+    Expect.equals("original error", ex);
+    called = true;
+    return true;
+  });
+
+  completer.completeException("original error");
+  Expect.isTrue(called);
+  Expect.isFalse(transformedFuture.isComplete);
+}
+
+testTransformOnExceptionReturnsNontrue() {
+  final completer = new Completer<String>();
+  var called = false;
+
+  final transformedFuture = completer.future.transform((x) {
+    Expect.fail("should not get here");
+  }, (ex) {
+    Expect.equals("original error", ex);
+    called = true;
+    return null;
+  });
+
+  completer.completeException("original error");
+  Expect.isTrue(called);
+  Expect.isTrue(transformedFuture.isComplete);
+  Expect.equals("original error", transformedFuture.exception);
+}
+
+
 // Tests for Future.chain
 
 testChainSuccess() {
   final completerA = new Completer<String>();
   final completerB = new Completer<String>();
   final chainedFuture = completerA.future.chain((x) {
     Expect.equals("42", x);
     return completerB.future;
   });
   Expect.isFalse(chainedFuture.isComplete);
@@ skipping 67 matching lines @@
   testCallStackIsNullIfCompletedSuccessfully();
   testCallStackReturnsCallstackPassedToCompleteException();
   testCallStackIsCapturedIfTransformCallbackThrows();
   testCallStackIsCapturedIfChainCallbackThrows();
   testCompleteWithCompletionAndSuccessHandlers();
   testExceptionWithCompletionAndSuccessHandlers();
   testExceptionWithCompletionAndSuccessAndExceptionHandlers();
   testTransformSuccess();
   testTransformFutureFails();
   testTransformTransformerFails();
+  testTransformOnExceptionTransformerThrows();
+  testTransformOnExceptionThrows();
+  testTransformOnExceptionReturnsTrue();
+  testTransformOnExceptionReturnsNontrue();
   testChainSuccess();
   testChainFirstFutureFails();
   testChainTransformerFails();
   testChainSecondFutureFails();
 }