[kernel] Fix assert message.

pkg/kernel/lib/binary/ast_to_binary.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_binary;
 
 import '../ast.dart';
 import '../import_table.dart';
 import 'tag.dart';
 import 'dart:convert';
 import 'dart:typed_data';
@@ skipping 849 matching lines @@
     _variableIndexer.popScope();
   }
 
   visitEmptyStatement(EmptyStatement node) {
     writeByte(Tag.EmptyStatement);
   }
 
   visitAssertStatement(AssertStatement node) {
     writeByte(Tag.AssertStatement);
     writeNode(node.condition);
+    writeOffset(node.conditionStartOffset);
+    writeOffset(node.conditionEndOffset);
     writeOptionalNode(node.message);
   }
 
   visitLabeledStatement(LabeledStatement node) {
     _labelIndexer.enter(node);
     writeByte(Tag.LabeledStatement);
     writeNode(node.body);
     _labelIndexer.exit();
   }
 
@@ skipping 583 matching lines @@
     _sink.add(_buffer.sublist(0, length));
     _buffer = new Uint8List(SIZE);
     flushedLength += length;
     length = 0;
   }
 
   void flushAndDestroy() {
     _sink.add(_buffer.sublist(0, length));
   }
 }