Fix analyzer warnings in assertions of the TypeExpression CL

pkg/compiler/lib/src/js_backend/codegen/codegen.dart

Index: pkg/compiler/lib/src/js_backend/codegen/codegen.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
index 7f176fd664ce3516d3a73108591a42cad60ca925..1815ee2a8b7905d76029859c51408e619a36cdf8 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
@@ -992,7 +992,8 @@ class CodeGenerator extends tree_ir.StatementVisitor
             node.dartType, arguments, registry);
       case tree_ir.TypeExpressionKind.INSTANCE:
         // We expect only flat types for the INSTANCE representation.
-        assert(node.dartType == node.dartType.element.thisType);
+        assert(node.dartType ==
+               (node.dartType.element as ClassElement).thisType);
         registry.registerInstantiatedClass(glue.listClass);
         return new js.ArrayInitializer(arguments);
     }