Move parser helper classes to own files and clean them up.

pkg/front_end/lib/src/fasta/parser/native_support.dart

Index: pkg/front_end/lib/src/fasta/parser/native_support.dart
diff --git a/pkg/front_end/lib/src/fasta/parser/native_support.dart b/pkg/front_end/lib/src/fasta/parser/native_support.dart
index 2959f7140fd65978bbe7e4a053513110237ec73b..13c44c4534505630a876ca332681f3b18982d09f 100644
--- a/pkg/front_end/lib/src/fasta/parser/native_support.dart
+++ b/pkg/front_end/lib/src/fasta/parser/native_support.dart
@@ -19,13 +19,13 @@ library fasta.parser.dart_vm_native;
 
 import '../../scanner/token.dart' show Token;
 
+import '../quote.dart' show unescapeString;
+
 import '../scanner/token_constants.dart' show STRING_TOKEN;
 
 import '../util/link.dart' show Link;
 
-import 'parser.dart' show optional;
-
-import '../quote.dart' show unescapeString;
+import 'util.dart' show optional;
 
 /// When parsing a library file, we may encounter a native clause
 /// instead of a function body. This method skips such a clause. The