Remove abstract from class members in dart libraries

sdk/lib/core/comparable.dart

 // Copyright (c) 2011, 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.
 
 // Dart core library.
 
 /**
  * The signature of a generic comparison function.
  *
  * A comparison function represents an ordering on a type of objects.
@@ skipping 13 matching lines @@
  */
 abstract class Comparable {
   /**
    * Compares this object to another [Comparable]
    *
    * Returns a value like a [Comparator] when comparing [:this:] to [other].
    *
    * May throw an [ArgumentError] if [other] is of a type that
    * is not comparable to [:this:].
    */
-  abstract int compareTo(Comparable other);
+  int compareTo(Comparable other);
 
   /**
    * Compare one comparable to another.
    *
    * This utility function is used as the default comparator
    * for the [List] sort function.
    */
   static int compare(Comparable a, Comparable b) => a.compareTo(b);
 }