Add AstBuilder support for import and export directives.

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

Index: pkg/front_end/lib/src/fasta/parser/listener.dart
diff --git a/pkg/front_end/lib/src/fasta/parser/listener.dart b/pkg/front_end/lib/src/fasta/parser/listener.dart
index ee9ea456b2aef2b94df4b2d6b589e5dff31dd0cf..738620faadd27cb6c3a4669e4a1188884f816640 100644
--- a/pkg/front_end/lib/src/fasta/parser/listener.dart
+++ b/pkg/front_end/lib/src/fasta/parser/listener.dart
@@ -116,6 +116,11 @@ class Listener {
 
   void beginExport(Token token) {}
 
+  /// Handle the end of an export directive.  Substructures:
+  /// - metadata
+  /// - uri
+  /// - conditional uris
+  /// - combinators
   void endExport(Token exportKeyword, Token semicolon) {
     logEvent("Export");
   }
@@ -236,6 +241,8 @@ class Listener {
 
   void beginHide(Token hideKeyword) {}
 
+  /// Handle the end of a "hide" combinator.  Substructures:
+  /// - hidden names (IdentifierList)
   void endHide(Token hideKeyword) {
     logEvent("Hide");
   }
@@ -272,6 +279,12 @@ class Listener {
 
   void beginImport(Token importKeyword) {}
 
+  /// Handle the end of an import directive.  Substructures:
+  /// - metadata
+  /// - uri
+  /// - conditional uris
+  /// - prefix identifier (only if asKeyword != null)
+  /// - combinators
   void endImport(Token importKeyword, Token DeferredKeyword, Token asKeyword,
       Token semicolon) {
     logEvent("Import");
@@ -457,6 +470,8 @@ class Listener {
 
   void beginShow(Token showKeyword) {}
 
+  /// Handle the end of a "show" combinator.  Substructures:
+  /// - shown names (IdentifierList)
   void endShow(Token showKeyword) {
     logEvent("Show");
   }