Use failedAt in more places (js_emitter)

pkg/compiler/lib/src/js_emitter/program_builder/field_visitor.dart

 // Copyright (c) 2015, 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.
 
 part of dart2js.js_emitter.program_builder;
 
 /**
  * [member] is a field (instance, static, or top level).
  *
  * [name] is the field name that the [Namer] has picked for this field's
@@ skipping 35 matching lines @@
    * [visitStatics] is false.
    *
    * If [element] is a [LibraryElement], [visitStatics] must be true.
    *
    * When visiting the instance fields of a class, the fields of its superclass
    * are also visited if the class is instantiated.
    *
    * Invariant: [element] must be a declaration element.
    */
   void visitFields(Element element, bool visitStatics, AcceptField f) {
-    assert(invariant(element, element.isDeclaration));
+    assert(element.isDeclaration, failedAt(element));
 
     ClassElement cls;
     bool isNativeClass = false;
     bool isLibrary = false;
     bool isInstantiated = false;
     if (element.isClass) {
       cls = element;
       isNativeClass = _nativeData.isNativeClass(cls);
 
       // If the class is never instantiated we still need to set it up for
       // inheritance purposes, but we can simplify its JavaScript constructor.
       isInstantiated =
           _codegenWorldBuilder.directlyInstantiatedClasses.contains(cls);
     } else if (element.isLibrary) {
       isLibrary = true;
-      assert(invariant(element, visitStatics));
+      assert(visitStatics, failedAt(element));
     } else {
       throw new SpannableAssertionFailure(
           element, 'Expected a ClassElement or a LibraryElement.');
     }
 
     void visitField(Element holder, FieldElement field) {
-      assert(invariant(element, field.isDeclaration));
+      assert(field.isDeclaration, failedAt(element));
 
       bool isMixinNativeField = isNativeClass && holder.isMixinApplication;
 
       // See if we can dynamically create getters and setters.
       // We can only generate getters and setters for [element] since
       // the fields of super classes could be overwritten with getters or
       // setters.
       bool needsGetter = false;
       bool needsSetter = false;
       if (isLibrary || isMixinNativeField || holder == element) {
@@ skipping 67 matching lines @@
         field is ClosureFieldElement;
   }
 
   bool canAvoidGeneratedCheckedSetter(VariableElement member) {
     // We never generate accessors for top-level/static fields.
     if (!member.isInstanceMember) return true;
     ResolutionDartType type = member.type;
     return type.treatAsDynamic || type.isObject;
   }
 }