Don't crash on error at end-of-file.

pkg/compiler/lib/src/io/source_file.dart

 // Copyright (c) 2012, 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 dart2js.io.source_file;
 
 import 'dart:math';
 import 'dart:convert' show UTF8;
 import 'dart:typed_data' show Uint8List;
 
@@ skipping 119 matching lines @@
    *
    * Use [colorize] to wrap source code text and marker characters in color
    * escape codes.
    */
   String getLocationMessage(String message, int start, int end,
                             {bool includeSourceLine: true,
                              String colorize(String text)}) {
     if (colorize == null) {
       colorize = (text) => text;
     }
+    if (end > length) {

[ahe, 2015/11/02 16:55:31]

Add a TODO to turn this into an assertion. If you get the wrong position from,
for example, a patch file this could be very confusing.

[ahe, 2015/11/02 16:56:02]

I meant to say: "consider adding a TODO..."

+      start = length - 1;
+      end = length;
+    }
+
     int lineStart = getLine(start);
     int columnStart = getColumn(lineStart, start);
     int lineEnd = getLine(end);
     int columnEnd = getColumn(lineEnd, end);
 
     StringBuffer buf = new StringBuffer('${filename}:');
     if (start != end || start != 0) {
       // Line/column info is relevant.
       buf.write('${lineStart + 1}:${columnStart + 1}:');
     }
@@ skipping 130 matching lines @@
   set length(int v) { }
 
   String slowText() => text;
 
   List<int> slowUtf8ZeroTerminatedBytes() {
     return _zeroTerminateIfNecessary(UTF8.encode(text));
   }
 
   String slowSubstring(int start, int end) => text.substring(start, end);
 }