[kernel] Fix assert message.

pkg/front_end/lib/src/fasta/kernel/body_builder.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 fasta.body_builder;
 
 import '../fasta_codes.dart'
     show
         FastaMessage,
         codeConstFieldWithoutInitializer,
@@ skipping 2639 matching lines @@
     // inInitializer should be false anyway.
     inInitializer = false;
   }
 
   @override
   void endAssert(Token assertKeyword, Assert kind, Token leftParenthesis,
       Token commaToken, Token rightParenthesis, Token semicolonToken) {
     debugEvent("Assert");
     Expression message = popForValueIfNotNull(commaToken);
     Expression condition = popForValue();
-    AssertStatement statement = new AssertStatement(condition, message);
+    AssertStatement statement = new AssertStatement(condition,
+        conditionStartOffset: leftParenthesis.offset + 1,
+        conditionEndOffset: rightParenthesis.offset,
+        message: message);
     switch (kind) {
       case Assert.Statement:
         push(statement);
         break;
 
       case Assert.Expression:
         push(buildCompileTimeError("`assert` can't be used as an expression."));
         break;
 
       case Assert.Initializer:
@@ skipping 996 matching lines @@
     if (starToken == null) {
       return AsyncMarker.Async;
     } else {
       assert(identical(starToken.stringValue, "*"));
       return AsyncMarker.AsyncStar;
     }
   } else {
     return internalError("Unknown async modifier: $asyncToken");
   }
 }