Use failedAt in more places (js_emitter)

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

Index: pkg/compiler/lib/src/js_emitter/program_builder/field_visitor.dart
diff --git a/pkg/compiler/lib/src/js_emitter/program_builder/field_visitor.dart b/pkg/compiler/lib/src/js_emitter/program_builder/field_visitor.dart
index 1e61b8a21c64473f804b085de2dc8bde3c072c46..92cfbd7241f4404edbae666d5028734ffbac59dc 100644
--- a/pkg/compiler/lib/src/js_emitter/program_builder/field_visitor.dart
+++ b/pkg/compiler/lib/src/js_emitter/program_builder/field_visitor.dart
@@ -53,7 +53,7 @@ class FieldVisitor {
    * 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;
@@ -69,14 +69,14 @@ class FieldVisitor {
           _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;