[V8] Use previous token location as EOS token location

src/messages.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // -------------------------------------------------------------------
 
 var kMessages = {
   // Error
   constructor_is_generator:      ["Class constructor may not be a generator"],
   constructor_is_accessor:       ["Class constructor may not be an accessor"],
@@ skipping 383 matching lines @@
  * @param {number} position The source position.
  * @return {number} 0 if input too small, -1 if input too large,
        else the line number.
  */
 function ScriptLineFromPosition(position) {
   var lower = 0;
   var upper = this.lineCount() - 1;
   var line_ends = this.line_ends;
 
   // We'll never find invalid positions so bail right away.
+  if (position - 1 > line_ends[upper]) {

[yurys, 2015/04/27 09:23:15]

I'd rather v8 highlighted position of the last token before EOS one. EOS is
oftentimes very confusing for the user as it gives little information about what
construction of the language is incomplete. Is there a way to provide position
of the token that goes before eos as the error position?

+    return -1;
+  }
+
+  // Process EOS token
   if (position > line_ends[upper]) {
-    return -1;
+    return upper;
   }
 
   // This means we don't have to safe-guard indexing line_ends[i - 1].
   if (position <= line_ends[0]) {
     return 0;
   }
 
   // Binary search to find line # from position range.
   while (upper >= 1) {
     var i = (lower + upper) >> 1;
@@ skipping 851 matching lines @@
 function SetUpStackOverflowBoilerplate() {
   var boilerplate = MakeRangeError(kStackOverflow);
 
   %DefineAccessorPropertyUnchecked(
       boilerplate, 'stack', StackTraceGetter, StackTraceSetter, DONT_ENUM);
 
   return boilerplate;
 }
 
 var kStackOverflowBoilerplate = SetUpStackOverflowBoilerplate();