Add a 'secure' constructor to Random in dart:math returning a cryptographically

tests/lib/math/random_secure_test.dart

+// Copyright (c) 2015, 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.
+
+// Library tag to allow Dartium to run the test.
+library random_secure;
+
+import "package:expect/expect.dart";
+import 'dart:math';
+
+main() {
+  var results = [];
+  for(var i = 0; i < 10; i++) {
+    var rng = new Random.secure();
+    var intVal = rng.nextInt(10000000);
+    Expect.isFalse(results.contains(intVal));

[Lasse Reichstein Nielsen, 2015/10/14 11:08:45]

This is not a test of randomness.
It's just a flaky test with a very long expected period between failures.
That is, if the test fails, it's not showing an actual error. Only if a long
sequence of test runs have too many failures is it a sign that the randomness
isn't random enough.

The odds of collisions in 10 elements out of 10'000'000 values is ... (looking
up birthday paradox) ... ~5e-7.

We won't be able to detect whether it fails significantly more often than once
per 2M runs, unless it is extremely often. If we see a failure, there is no
rational action to take, so the failure will be ignored and so the test might as
well not be here.

I'm not sure what is a good way to test randomness (some proper randomness tests
testing specific properties of the distribution over a long period might work),
but being random is about lacking some properties in general (and
cryptographically secure even more so), but any test will fail eventually for a
specific run of any proper random generator.

[regis, 2015/10/14 20:22:36]

Acknowledged.
I just copied code from random_big_test.dart, which is even more flaky, given
the smaller max.
The idea here is only to detect a totally broken implementation that would
always return the same value.

+    results.add(intVal);
+    var doubleVal = rng.nextDouble();
+    Expect.isFalse(results.contains(doubleVal));

[Lasse Reichstein Nielsen, 2015/10/14 11:08:46]

I wouldn't check doubles in the same loop as the integers, or even on the same
RNG.

[regis, 2015/10/14 20:22:36]

Done.

+    results.add(doubleVal);
+    rng.nextBool();
+  }

[Lasse Reichstein Nielsen, 2015/10/14 11:08:46]

Basically, there is no need to make a new test. Use whatever test we already
have for the non-CS RNG and run it on the CSRNG as well.
If it was good enough before, then it probably still is (or at least it's no
worse).

[regis, 2015/10/14 20:22:35]

One existing test checks that the sequence is deterministic given the same seed
(not applicable here). The other test (that I copied from) checks that a big
seed is allowed (no seed here). 

I prefer to have a new test, so I can mark it as failing for dart2js. Making the
existing one a multi-test requires even more fiddling with the status files.

I added a comment to the new test explaining that it is flaky by definition, but
still better than no test.

+}