Add some documentation to the parser.

dart/lib/compiler/implementation/scanner/listener.dart

Index: dart/lib/compiler/implementation/scanner/listener.dart
diff --git a/dart/lib/compiler/implementation/scanner/listener.dart b/dart/lib/compiler/implementation/scanner/listener.dart
index f80ef8ebac15b21d98f88e35480243dd63bbac22..cb986b4c08dd3bb63de7d14563f296751fc893ac 100644
--- a/dart/lib/compiler/implementation/scanner/listener.dart
+++ b/dart/lib/compiler/implementation/scanner/listener.dart
@@ -4,6 +4,10 @@
 
 final bool VERBOSE = false;
 
+/**
+ * A parser event listener that does nothing except throw exceptions
+ * on parser errors.
+ */
 class Listener {
   void beginArguments(Token token) {
   }
@@ -490,7 +494,10 @@ class ParserError {
 }
 
 /**
- * A listener for parser events.
+ * A parser event listener designed to work with [PartialParser]. It
+ * builds elements representing the top-level declarations found in
+ * the parsed compilation unit and records them in
+ * [compilationUnitElement].
  */
 class ElementListener extends Listener {
   final DiagnosticListener listener;