Fix DoubleLinkedListQueue misbehavior on concurrent modification.

sdk/lib/core/num.dart

Index: sdk/lib/core/num.dart
diff --git a/sdk/lib/core/num.dart b/sdk/lib/core/num.dart
index 21f23eef18321ab7fc4b59aa397cdbb7ead0b0d6..54abff3bfa3c47a52dc7983027c23482169a4026 100644
--- a/sdk/lib/core/num.dart
+++ b/sdk/lib/core/num.dart
@@ -51,7 +51,7 @@ abstract class num implements Comparable<num> {
    * for an [int] and a [double] with the same numerical value, and therefore
    * the same value for the doubles zero and minus zero.
    *
-   * No guarantees are made about the hash code of NaN.
+   * No guarantees are made about the hash code of NaN values.
    */
   int get hashCode;