Don't inherit two versions of Comparable. Not supported by dart2js.

tests/corelib/map_test.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.
 
 library map_test;
 import "package:expect/expect.dart";
 import 'dart:collection';
 import 'dart:convert' show JSON;
 
 Map newJsonMap()
@@ skipping 885 matching lines @@
     int index = _keys.indexOf(key);
     if (index < 0) return null;
     return _values[index];
   }
 
   Iterable<K> get keys => _keys.skip(0);
 }
 
 abstract class Super implements Comparable {}
 abstract class Interface implements Comparable {}
-class Sub extends Super implements Interface, Comparable<Sub> {
+class Sub extends Super implements Interface, Comparable {
   int compareTo(Sub other) => 0;
   int get hashCode => 0;
-  bool operator==(Object other) => other is Sub;
+  bool operator==(other) => other is Sub;
 }
 
 expectMap(Map expect, Map actual) {
   Expect.equals(expect.length, actual.length, "length");
   for (var key in expect.keys) {
     Expect.isTrue(actual.containsKey(key), "containsKey $key");
     Expect.equals(expect[key], actual[key]);
   }
 }
 
@@ skipping 22 matching lines @@
   Map<Interface, Interface> interfaceMap = <Interface, Interface>{sub: sub};
   expectMap(superMap, new Map<Super, Super>.from(interfaceMap));
   expectMap(superMap, new Map<Interface, Interface>.from(superMap));
   expectMap(superMap, new HashMap<Super, Super>.from(interfaceMap));
   expectMap(superMap, new HashMap<Interface, Interface>.from(superMap));
   expectMap(superMap, new LinkedHashMap<Super, Super>.from(interfaceMap));
   expectMap(superMap, new LinkedHashMap<Interface, Interface>.from(superMap));
   expectMap(superMap, new SplayTreeMap<Super, Super>.from(interfaceMap));
   expectMap(superMap, new SplayTreeMap<Interface, Interface>.from(superMap));
 }