Use failedAt in more places (misc2)

pkg/compiler/lib/src/elements/elements.dart

Index: pkg/compiler/lib/src/elements/elements.dart
diff --git a/pkg/compiler/lib/src/elements/elements.dart b/pkg/compiler/lib/src/elements/elements.dart
index 906db4ffba55e2696647909026b288581816550e..87478cc740f3514f11a34a6809b90ce6a3e4c449 100644
--- a/pkg/compiler/lib/src/elements/elements.dart
+++ b/pkg/compiler/lib/src/elements/elements.dart
@@ -826,7 +826,7 @@ class Elements {
       FunctionElement element,
       T compileArgument(Node argument),
       T compileDefaultValue(ParameterElement element)) {
-    assert(invariant(element, element.isImplementation));
+    assert(element.isImplementation, failedAt(element));
     List<T> result = <T>[];
 
     FunctionSignature parameters = element.functionSignature;