Use failedAt in more places

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

Index: pkg/compiler/lib/src/js_backend/element_strategy.dart
diff --git a/pkg/compiler/lib/src/js_backend/element_strategy.dart b/pkg/compiler/lib/src/js_backend/element_strategy.dart
index f8c9a276b5a3e601306c9fcc1e6ef0da7cfb3652..bd682929c9f1838f95446d17691511d6887b135c 100644
--- a/pkg/compiler/lib/src/js_backend/element_strategy.dart
+++ b/pkg/compiler/lib/src/js_backend/element_strategy.dart
@@ -79,7 +79,7 @@ class ElementCodegenWorkItemBuilder extends WorkItemBuilder {
 
   @override
   WorkItem createWorkItem(MemberElement element) {
-    assert(invariant(element, element.isDeclaration));
+    assert(element.isDeclaration, failedAt(element));
     // Don't generate code for foreign elements.
     if (_backend.isForeign(element)) return null;
     if (element.isAbstract) return null;
@@ -108,8 +108,8 @@ class ElementCodegenWorkItem extends CodegenWorkItem {
     // missing call of form registry.registerXXX. Alternatively, the code
     // generation could spuriously be adding dependencies on things we know we
     // don't need.
-    assert(invariant(element, element.hasResolvedAst,
-        message: "$element has no resolved ast."));
+    assert(element.hasResolvedAst,
+        failedAt(element, "$element has no resolved ast."));
     ResolvedAst resolvedAst = element.resolvedAst;
     return new ElementCodegenWorkItem.internal(
         resolvedAst, backend, closedWorld);