Ensure hash pattern is non-zero. Re-enable compact hash map/set.

tests/corelib/hash_set_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.
 //
 // VMOptions=
 // VMOptions=--use_compact_hash=false
 
 // Tests of hash set behavior, with focus in iteration and concurrent
 // modification errors.
 
@@ skipping 236 matching lines @@
     set.remove(m1a);
     set.add(m1b);
     Expect.identical(m1b, set.lookup(m1a));
     Expect.identical(m1b, set.lookup(m1b));
 
     set.add(1);
     Expect.identical(1, set.lookup(1.0));
     set.add(-0.0);
     Expect.identical(-0.0, set.lookup(0.0));
   }
+
+  {  // Test special hash codes
+    Set set = newSet();
+    List keys = [];
+    // Powers of two
+    for (int i = 65; i >= 2; --i) {
+      keys.add(new Mutable(1 << i));
+    }
+    for (var key in keys) {
+      Expect.isTrue(set.add(key));
+    }
+    for (var key in keys) {
+      Expect.isTrue(set.contains(key));
+    }
+  }
 }
 
 
 void testIdentitySet(Set create()) {
   Set set = create();
   set.add(1);
   set.add(2);
   set.add(1);  // Integers are identical if equal.
   Expect.equals(2, set.length);
   var complex = 4;
@@ skipping 58 matching lines @@
   // Can't make a bad compareTo that isn't invalid.
   int compareTo(BadHashCode other) => id - other.id;
 }
 
 class Mutable {
   int id;
   Mutable(this.id);
   int get hashCode => id;
   bool operator==(other) => other is Mutable && id == other.id;
 }