Make Comparable 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 e544201dca6e56ef4ff914b0a6ad6dc25f4dd696..7cc51837481de3ae705b3da9917624e322dd5773 100644
--- a/sdk/lib/core/comparable.dart
+++ b/sdk/lib/core/comparable.dart
@@ -22,7 +22,7 @@ typedef int Comparator<T>(T a, T b);
 /**
  * Interface used by types that have an intrinsic ordering.
  */
-abstract class Comparable {
+abstract class Comparable<T extends Comparable<T>> {
   /**
    * Compares this object to another [Comparable]
    *
@@ -31,7 +31,7 @@ abstract class Comparable {
    * May throw an [ArgumentError] if [other] is of a type that
    * is not comparable to [:this:].
    */
-  int compareTo(Comparable other);
+  int compareTo(T other);
 
   /**
    * Compare one comparable to another.