dart2js-kernel: Handle some more 'is' cases

pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart

Index: pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
diff --git a/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart b/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
index fd59cd5759fd407d84d403a5f1ee9a7e7fb1173f..a392ec8bd077f5545fdb1d39eb9e5b080d4acce2 100644
--- a/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
+++ b/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
@@ -326,6 +326,12 @@ class KernelAstAdapter {
       TypeMaskFactory.inferredReturnTypeForElement(
           _backend.helpers.checkConcurrentModificationError, _compiler);
 
+  ir.Procedure get checkSubtype =>
+      kernel.functions[_backend.helpers.checkSubtype];
+
+  ir.Procedure get checkSubtypeOfRuntimeType =>
+      kernel.functions[_backend.helpers.checkSubtypeOfRuntimeType];
+
   ir.Procedure get assertHelper =>
       kernel.functions[_backend.helpers.assertHelper];