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

tests/lib/math/random_test.dart

 // Copyright (c) 2013, 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.
 
 // Test that rnd.nextInt with a seed generates the same sequence each time.
 
 // Library tag to allow Dartium to run the test.
 library random_test;
 
 import "package:expect/expect.dart";
@@ skipping 38 matching lines @@
   Expect.equals(   9680425, rnd.nextInt(i *= 2));
   Expect.equals(  28460171, rnd.nextInt(i *= 2));
   Expect.equals(  49481738, rnd.nextInt(i *= 2));
   Expect.equals(   9878974, rnd.nextInt(i *= 2));
   Expect.equals( 132552472, rnd.nextInt(i *= 2));
   Expect.equals( 210267283, rnd.nextInt(i *= 2));
   Expect.equals( 125422442, rnd.nextInt(i *= 2));
   Expect.equals( 226275094, rnd.nextInt(i *= 2));
   Expect.equals(1639629168, rnd.nextInt(i *= 2));
   Expect.equals(0x100000000, i);
+  Expect.equals( 1552504971, rnd.nextInt(i));

[Lasse Reichstein Nielsen, 2015/10/15 09:32:33]

Why? We already called with that value of max above.

[regis, 2015/10/15 17:12:02]

Aargh. Left over from debugging. Reverted.

I was trying to figure out why is this test not failing, since we have the wrong
check in nextInt:
(rnd32 - result + max) >= _POW2_32
instead of
(rnd32 - result + max) > _POW2_32

The reason it succeeds is that we use another code path if max is a power of 2.

   // If max is too large expect an ArgumentError.
   Expect.throws(() => rnd.nextInt(i + 1), (e) => e is ArgumentError);
 
   rnd = new Random(6790);
   Expect.approxEquals(0.1202733131, rnd.nextDouble());
   Expect.approxEquals(0.5554054805, rnd.nextDouble());
   Expect.approxEquals(0.0385160727, rnd.nextDouble());
   Expect.approxEquals(0.2836345217, rnd.nextDouble());
 }
 
@@ skipping 23 matching lines @@
   ];
   for (var i = 0, m = 1; i < 75; i++) {
     Expect.equals(expectations[i], new Random(rawSeed * m).nextInt(65536));
     Expect.equals(negative_seed_expectations[i],
                   new Random(rawSeed * -m).nextInt(65536));
     m *= 2;
   }
   // And test zero seed too.
   Expect.equals(21391, new Random(0).nextInt(65536));
 }