Use failedAt in more places (js_emitter)

pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart

Index: pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart
diff --git a/pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart b/pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart
index 200e5ad8b5261b0c3466402f987cf5681ee6c54b..3d12635d1bdc878719eaf802db8942c5af6c14e9 100644
--- a/pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart
+++ b/pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart
@@ -99,7 +99,7 @@ class RuntimeTypeGenerator {
   /// type variables.
   TypeTestProperties generateIsTests(ClassElement classElement,
       {bool storeFunctionTypeInMetadata: true}) {
-    assert(invariant(classElement, classElement.isDeclaration));
+    assert(classElement.isDeclaration, failedAt(classElement));
 
     TypeTestProperties result = new TypeTestProperties();