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 2fb66213046ec6d5a05e6d6cf3e18b2389269ecd..2013c476600fad79b522c9ae39a8c333f340edb0 100644
--- a/pkg/compiler/lib/src/js_backend/element_strategy.dart
+++ b/pkg/compiler/lib/src/js_backend/element_strategy.dart
@@ -67,7 +67,7 @@ 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;
@@ -96,8 +96,8 @@ 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);