Use failedAt in more places

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

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.js_backend.element_strategy;
 
 import '../backend_strategy.dart';
 import '../common.dart';
 import '../common/codegen.dart';
 import '../common/work.dart';
@@ skipping 49 matching lines @@
 class ElementCodegenWorkItemBuilder extends WorkItemBuilder {
   final JavaScriptBackend _backend;
   final ClosedWorld _closedWorld;
   final CompilerOptions _options;
 
   ElementCodegenWorkItemBuilder(
       this._backend, this._closedWorld, this._options);
 
   @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;
 
     // Codegen inlines field initializers. It only needs to generate
     // code for checked setters.
     if (element.isField && element.isInstanceMember) {
       if (!_options.enableTypeAssertions ||
           element.enclosingElement.isClosure) {
         return null;
       }
     }
     return new ElementCodegenWorkItem(_backend, _closedWorld, element);
   }
 }
 
 class ElementCodegenWorkItem extends CodegenWorkItem {
   CodegenRegistry registry;
   final ResolvedAst resolvedAst;
   final JavaScriptBackend _backend;
   final ClosedWorld _closedWorld;
 
   factory ElementCodegenWorkItem(JavaScriptBackend backend,
       ClosedWorld closedWorld, MemberElement element) {
     // If this assertion fails, the resolution callbacks of the backend may be
     // 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);
   }
 
   ElementCodegenWorkItem.internal(
       this.resolvedAst, this._backend, this._closedWorld);
 
   MemberElement get element => resolvedAst.element;
 
   WorldImpact run() {
     registry = new CodegenRegistry(element);
     return _backend.codegen(this, _closedWorld);
   }
 
   String toString() => 'CodegenWorkItem(${resolvedAst.element})';
 }