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

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

Index: pkg/front_end/lib/src/fasta/parser/class_member_parser.dart
diff --git a/pkg/front_end/lib/src/fasta/parser/class_member_parser.dart b/pkg/front_end/lib/src/fasta/parser/class_member_parser.dart
index 22e27d40b2e064d3da2ce5ebd7c3db2ddcae68cd..52d1eec14fd78e63546486e74b78d48adf7c51b6 100644
--- a/pkg/front_end/lib/src/fasta/parser/class_member_parser.dart
+++ b/pkg/front_end/lib/src/fasta/parser/class_member_parser.dart
@@ -8,9 +8,11 @@ import '../../scanner/token.dart' show Token;
 
 import '../fasta_codes.dart' show Message;
 
+import 'assert.dart' show Assert;
+
 import 'listener.dart' show Listener;
 
-import 'parser.dart' show Assert, Parser;
+import 'parser.dart' show Parser;
 
 /// Parser similar to [TopLevelParser] but also parses class members (excluding
 /// their bodies).