Use failedAt in more places

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

Index: pkg/compiler/lib/src/elements/modelx.dart
diff --git a/pkg/compiler/lib/src/elements/modelx.dart b/pkg/compiler/lib/src/elements/modelx.dart
index 2f40c9213783d019ba5029419a4cf0b194f33eae..30c51e82ccfe6ebdccdb34902c5879c14e8c15cc 100644
--- a/pkg/compiler/lib/src/elements/modelx.dart
+++ b/pkg/compiler/lib/src/elements/modelx.dart
@@ -1570,8 +1570,9 @@ abstract class VariableElementX extends ElementX
         definitionCache = initializedIdentifier;
       }
     }
-    invariant(definitions, definitionCache != null,
-        message: "Could not find '$name'.");
+    if (definitionCache == null) {
+      failedAt(definitionCache, "Could not find '$name'.");
+    }
     definitionsCache = definitions;
   }