[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 2640 matching lines @@
     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);
+    statement.conditionStartOffset = leftParenthesis.next.offset;
+    Token lastTokenInCondition = rightParenthesis.previous;

[ahe, 2017/06/16 07:39:42]

We want to get rid of "previous". Could you do something like this instead:

Token lastTokenInCondition = leftParenthesis;
while (lastTokenInCondition.next != rightParenthesis) {
  lastTokenInCondition = lastTokenInCondition.next;
}

Or:

Token lastTokenInCondition = rightParenthesis.offset - 1;

[jensj, 2017/06/16 07:43:29]

I could. But:
1) I suppose an assert can be arbitrarily long so that could potentially be
slow.
2) This could theoretically include a lot of linebreaks and whatnot at the end,
when we want to extract the source code between the start and the end. It might
not be a big deal through, I'm not sure.

What do you think?

[ahe, 2017/06/16 07:53:35]

statement.conditionStartOffset = leftParenthesis.offset + 1;
statement.conditionEndOffset = rightParenthesis.offset - 1;

This way, you also get any comments included, and line breaks could easily be
removed with a call to "trim()" in runtime/lib/errors_patch.dart.

[jensj, 2017/06/16 09:28:25]

Actually not -1 on the end as that would could off the last char.

+    statement.conditionEndOffset =
+        lastTokenInCondition.offset + lastTokenInCondition.length;
     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");
   }
 }