Add some documentation to the parser.

dart/lib/compiler/implementation/scanner/parser.dart

Index: dart/lib/compiler/implementation/scanner/parser.dart
diff --git a/dart/lib/compiler/implementation/scanner/parser.dart b/dart/lib/compiler/implementation/scanner/parser.dart
index 00ccaa2c81acea9ec55bbd237cfd4ee3a4da6b81..0ecdc056103da55345392b4f2ad5882f4d54c668 100644
--- a/dart/lib/compiler/implementation/scanner/parser.dart
+++ b/dart/lib/compiler/implementation/scanner/parser.dart
@@ -4,7 +4,12 @@
 
 /**
  * An event generating parser of Dart programs. This parser expects
- * all tokens in a linked list.
+ * all tokens in a linked list (aka a token stream).
+ *
+ * The class [Scanner] is used to generate a token stream. See the
+ * file scanner.dart.
+ *
+ * Subclasses of the class [Listener] are used to listen to events.
  */
 class Parser {
   final Listener listener;