Use failedAt in more places

pkg/compiler/lib/src/js_backend/backend_usage.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.
 
 import '../common.dart';
 import '../common_elements.dart';
 import '../elements/elements.dart' show Element;
 import '../elements/entities.dart';
 import '../elements/types.dart';
 import '../util/util.dart' show Setlet;
@@ skipping 104 matching lines @@
   /// `true` if `Function.apply` is used.
   bool isFunctionApplyUsed = false;
 
   /// `true` if `noSuchMethod` is used.
   bool isNoSuchMethodUsed = false;
 
   BackendUsageBuilderImpl(this._commonElements);
 
   @override
   void registerBackendFunctionUse(FunctionEntity element) {
-    assert(invariant(element, _isValidBackendUse(element),
-        message: "Backend use of $element is not allowed."));
+    assert(_isValidBackendUse(element),
+        failedAt(element, "Backend use of $element is not allowed."));
     _helperFunctionsUsed.add(element);
   }
 
   @override
   void registerBackendClassUse(ClassEntity element) {
-    assert(invariant(element, _isValidBackendUse(element),
-        message: "Backend use of $element is not allowed."));
+    assert(_isValidBackendUse(element),
+        failedAt(element, "Backend use of $element is not allowed."));
     _helperClassesUsed.add(element);
   }
 
   bool _isValidBackendUse(Entity element) {
     if (_isValidEntity(element)) return true;
     if (element is Element) {
-      assert(invariant(element, element.isDeclaration,
-          message: "Backend use $element must be the declaration."));
+      assert(element.isDeclaration,
+          failedAt(element, "Backend use $element must be the declaration."));
       if (element.implementationLibrary.isPatch ||
           // Needed to detect deserialized injected elements, that is
           // element declared in patch files.
           (element.library.isPlatformLibrary &&
               element.sourcePosition.uri.path
                   .contains('_internal/js_runtime/lib/')) ||
           element.library == _commonElements.jsHelperLibrary ||
           element.library == _commonElements.interceptorsLibrary ||
           element.library == _commonElements.isolateHelperLibrary) {
         // TODO(johnniwinther): We should be more precise about these.
@@ skipping 190 matching lines @@
       _globalFunctionDependencies ?? const <FunctionEntity>[];
 
   @override
   Iterable<ClassEntity> get globalClassDependencies =>
       _globalClassDependencies ?? const <ClassEntity>[];
 
   Iterable<FunctionEntity> get helperFunctionsUsed => _helperFunctionsUsed;
 
   Iterable<ClassEntity> get helperClassesUsed => _helperClassesUsed;
 }