Make Comparable.compare generic.

sdk/lib/core/comparable.dart

Index: sdk/lib/core/comparable.dart
diff --git a/sdk/lib/core/comparable.dart b/sdk/lib/core/comparable.dart
index 7a22948a080fabb5649088b32bcfc4499fcb89c2..449d5ea3b8b10bccb23e762eb1615c26399eefa8 100644
--- a/sdk/lib/core/comparable.dart
+++ b/sdk/lib/core/comparable.dart
@@ -90,5 +90,5 @@ abstract class Comparable<T> {
    * This utility function is used as the default comparator
    * for ordering collections, for example in the [List] sort function.
    */
-  static int compare(Comparable a, Comparable b) => a.compareTo(b);
+  static int compare<T extends Comparable<T>>(T a, T b) => a.compareTo(b);
 }