Make the loopback variable public in expression builder (#168)

* Make the loopback variable public in expression builder

* Make the loopback variable in ExpressionBuilder a getter

* Add the get keyword

lib/src/expression/builder.dart

@@ -71,13 +71,17 @@ class ExpressionBuilder<T> {
   final List<ExpressionGroup<T>> _groups = [];
   final SettableParser<T> _loopback = undefined();
 
+  /// The parser for this expression builder. Can be used to loop back to this
+  /// parser.
+  SettableParser<T> get loopback => _loopback;
+
   /// Defines a new primitive, literal, or value [parser].
   void primitive(Parser<T> parser) => _primitives.add(parser);
 
   /// Creates a new group of operators that share the same priority.
   @useResult
   ExpressionGroup<T> group() {
-    final group = ExpressionGroup<T>(_loopback);
+    final group = ExpressionGroup<T>(loopback);
     _groups.add(group);
     return group;
   }
@@ -94,15 +98,15 @@ class ExpressionBuilder<T> {
       buildChoice(primitives),
       (parser, group) => group.build(parser),
     );
-    // Replace all uses of `_loopback` with `parser`. Do not use `resolve()`
+    // Replace all uses of `loopback` with `parser`. Do not use `resolve()`
     // because that might try to resolve unrelated parsers outside of the scope
     // of the `ExpressionBuilder` and cause infinite recursion.
     for (final parent in allParser(parser)) {
-      parent.replace(_loopback, parser);
+      parent.replace(loopback, parser);
     }
     // Also update the loopback parser, just in case somebody keeps a reference
     // to it (not that anybody should do that).
-    _loopback.set(parser);
+    loopback.set(parser);
     return parser;
   }
 }