Re-enable kernel unit tests on platforms where they are supported.

pkg/kernel/test/uint31_pair_map_test.dart

 import 'dart:math';
 import 'package:kernel/type_propagation/canonicalizer.dart';
 import 'package:test/test.dart';
 
 Random random = new Random(12345);
 
 main() {
   test('Uint31PairMap randomized tests', runTest);
 }
 
@@ skipping 13 matching lines @@
       int candidateValue = candidate.lookup(x, y);
       expect(candidateValue, equals(trustedValue));
       if (trustedValue == null) {
         int newValue = nextValue++;
         trusted[key] = newValue;
         candidate.put(newValue);
       }
     }
   }
 }