[kernel] Fix assert message.

pkg/kernel/lib/clone.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.clone;
 
 import 'ast.dart';
 import 'type_algebra.dart';
 
 /// Visitor that return a clone of a tree, maintaining references to cloned
 /// objects.
@@ skipping 252 matching lines @@
   visitBlock(Block node) {
     return new Block(node.statements.map(clone).toList());
   }
 
   visitEmptyStatement(EmptyStatement node) {
     return new EmptyStatement();
   }
 
   visitAssertStatement(AssertStatement node) {
     return new AssertStatement(
-        clone(node.condition), cloneOptional(node.message));
+        clone(node.condition), cloneOptional(node.message))
+      ..conditionStartOffset = node.conditionStartOffset
+      ..conditionEndOffset = node.conditionEndOffset;
   }
 
   visitLabeledStatement(LabeledStatement node) {
     LabeledStatement newNode = new LabeledStatement(null);
     labels[node] = newNode;
     newNode.body = clone(node.body)..parent = newNode;
     return newNode;
   }
 
   visitBreakStatement(BreakStatement node) {
@@ skipping 145 matching lines @@
   visitArguments(Arguments node) {
     return new Arguments(node.positional.map(clone).toList(),
         types: node.types.map(visitType).toList(),
         named: node.named.map(clone).toList());
   }
 
   visitNamedExpression(NamedExpression node) {
     return new NamedExpression(node.name, clone(node.value));
   }
 }