Print <<NULL>> instead of crashing.

pkg/kernel/lib/text/ast_to_text.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 library kernel.ast_to_text;
 
 import '../ast.dart';
 import '../import_table.dart';
 import '../type_propagation/type_propagation.dart';
 
 class Namer<T> {
@@ skipping 375 matching lines @@
     if (state == WORD) {
       writeSpace();
     }
   }
 
   void writeIndentation() {
     writeSpace('  ' * indentation);
   }
 
   void writeNode(Node node) {
-    node.accept(this);
+    if (node == null) {
+      writeSymbol("<Null>");
+    } else {
+      node.accept(this);
+    }
   }
 
   void writeOptionalNode(Node node) {
     if (node != null) {
       node.accept(this);
     }
   }
 
   void writeAnnotatedType(DartType type, String annotation) {
     writeType(type);
@@ skipping 1131 matching lines @@
   }
   throw 'illegal ProcedureKind: $kind';
 }
 
 class ExpressionPrinter {
   final Printer writeer;
   final int minimumPrecedence;
 
   ExpressionPrinter(this.writeer, this.minimumPrecedence);
 }