Implement AstBuilder support for `void` keyword.

pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart

Index: pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart b/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
index f47af376b5a337d057633c3854eab95f62c2eddd..08f97bf726f49472f268fcda4b930791447a1652 100644
--- a/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
+++ b/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
@@ -1066,6 +1066,16 @@ class AstBuilder extends ScopeListener {
   void endMember() {
     debugEvent("Member");
   }
+
+  @override
+  void handleVoidKeyword(Token token) {
+    debugEvent("VoidKeyword");
+    // TODO(paulberry): is this sufficient, or do we need to hook the "void"
+    // keyword up to an element?
+    handleIdentifier(token);
+    handleNoTypeArguments(token);
+    endType(token, token);
+  }
 }
 
 /// Data structure placed on the stack to represent a class body.