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

tests/corelib/map_test.dart

Index: tests/corelib/map_test.dart
diff --git a/tests/corelib/map_test.dart b/tests/corelib/map_test.dart
index efe6239712bea997fe879b0bdaeb80761cb9c9f5..79d1ace519b3fb3b0cfada9be45aefa09e513916 100644
--- a/tests/corelib/map_test.dart
+++ b/tests/corelib/map_test.dart
@@ -903,10 +903,10 @@ class UnmodifiableMapBaseMap<K, V> extends UnmodifiableMapBase<K, V> {
 
 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) {