Use failedAt in more places

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 2a32c5a71273f51a05178b6aaf1e949fddc26e14..f85f1a38c801632c0db932f3fe1ad1d4a545a0d0 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
@@ -79,7 +79,7 @@ class FieldVisitor {
       isLibrary = true;
       assert(visitStatics, failedAt(library));
     } else {
-      throw new SpannableAssertionFailure(NO_LOCATION_SPANNABLE,
+      failedAt(NO_LOCATION_SPANNABLE,
           'Expected a ClassElement or a LibraryElement.');
     }