Remove an unneeded ignore comment in dev_compiler

Change-Id: I90d4ee37e1599ad6981b379ca35c282efef923eb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/238982
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>
diff --git a/pkg/dev_compiler/lib/src/js_ast/js_ast.dart b/pkg/dev_compiler/lib/src/js_ast/js_ast.dart
index 8cb165e..d059a14 100644
--- a/pkg/dev_compiler/lib/src/js_ast/js_ast.dart
+++ b/pkg/dev_compiler/lib/src/js_ast/js_ast.dart
@@ -5,7 +5,6 @@
 // @dart = 2.9
 
 // ignore_for_file: directives_ordering
-// ignore_for_file: library_prefixes
 
 library js_ast;