[kernel] Fix assert message.

pkg/kernel/lib/binary/ast_from_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_from_binary;
 
 import 'dart:convert';
 import 'dart:typed_data';
 
 import '../ast.dart';
 import '../transformations/flags.dart';
@@ skipping 917 matching lines @@
     switch (tag) {
       case Tag.InvalidStatement:
         return new InvalidStatement();
       case Tag.ExpressionStatement:
         return new ExpressionStatement(readExpression());
       case Tag.Block:
         return readBlock();
       case Tag.EmptyStatement:
         return new EmptyStatement();
       case Tag.AssertStatement:
-        return new AssertStatement(readExpression(), readExpressionOption());
+        Expression condition = readExpression();
+        int conditionStart = readOffset();
+        int conditionEnd = readOffset();
+        return new AssertStatement(condition, readExpressionOption())
+          ..conditionStartOffset = conditionStart
+          ..conditionEndOffset = conditionEnd;
       case Tag.LabeledStatement:
         var label = new LabeledStatement(null);
         labelStack.add(label);
         label.body = readStatement()..parent = label;
         labelStack.removeLast();
         return label;
       case Tag.BreakStatement:
         int offset = readOffset();
         int index = readUInt();
         return new BreakStatement(labelStack[labelStackBase + index])
@@ skipping 248 matching lines @@
       ..fileOffset = offset
       ..fileEqualsOffset = fileEqualsOffset;
   }
 
   int readOffset() {
     // Offset is saved as unsigned,
     // but actually ranges from -1 and up (thus the -1)
     return readUInt() - 1;
   }
 }