Use entities in BackendClasses

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 8a4edae6ec05ca317ab1c17694e33b00d343fd38..7ee6e778ba93b21f51d842ca73a40b475a7dfc6a 100644
--- a/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
+++ b/pkg/compiler/lib/src/ssa/kernel_ast_adapter.dart
@@ -303,8 +303,8 @@ class KernelAstAdapter implements KernelElementAdapter {
       return true;
     }
     // TODO(sra): Recognize any combination of fixed length indexables.
-    if (mask.containsOnly(closedWorld.backendClasses.fixedListImplementation) ||
-        mask.containsOnly(closedWorld.backendClasses.constListImplementation) ||
+    if (mask.containsOnly(closedWorld.backendClasses.fixedListClass) ||
+        mask.containsOnly(closedWorld.backendClasses.constListClass) ||
         mask.containsOnlyString(closedWorld) ||
         closedWorld.commonMasks.isTypedArray(mask)) {
       return true;