Move ElementEnvironment.isSubtype to DartTypes

pkg/compiler/lib/src/kernel/element_map_impl.dart

Index: pkg/compiler/lib/src/kernel/element_map_impl.dart
diff --git a/pkg/compiler/lib/src/kernel/element_map_impl.dart b/pkg/compiler/lib/src/kernel/element_map_impl.dart
index 5a0ab99bc7ed3cd42765448fb01b9cae7ebdf1a6..fe16c051c18c3656c91f8aa48b77de7040e17aa9 100644
--- a/pkg/compiler/lib/src/kernel/element_map_impl.dart
+++ b/pkg/compiler/lib/src/kernel/element_map_impl.dart
@@ -950,11 +950,6 @@ class KernelElementEnvironment implements ElementEnvironment {
   }
 
   @override
-  bool isSubtype(DartType a, DartType b) {
-    return elementMap.types.isSubtype(a, b);
-  }
-
-  @override
   FunctionType getFunctionType(KFunction function) {
     return elementMap._getFunctionType(function);
   }