Library privacy.

dart/frog/leg/scanner/token.dart

Index: dart/frog/leg/scanner/token.dart
diff --git a/dart/frog/leg/scanner/token.dart b/dart/frog/leg/scanner/token.dart
index 2e4676d2e840c67fad9f6526575b325e3a0f4e53..fd2138d70da0789bb587856f97506a40bee4a20a 100644
--- a/dart/frog/leg/scanner/token.dart
+++ b/dart/frog/leg/scanner/token.dart
@@ -140,6 +140,8 @@ interface SourceString extends Hashable, Iterable<int> default StringWrapper {
   String get stringValue();
 
   bool isEmpty();
+
+  bool isPrivate();
 }
 
 class StringWrapper implements SourceString {
@@ -172,6 +174,8 @@ class StringWrapper implements SourceString {
   }
 
   bool isEmpty() => stringValue.isEmpty();
+
+  bool isPrivate() => !isEmpty() && stringValue.charCodeAt(0) === $_;
 }
 
 class StringCodeIterator implements Iterator<int> {