Purge String operator+ from dart2js.

dart/lib/compiler/implementation/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('source_file');
 
 #import('colors.dart', prefix: 'colors');
 
 /**
  * Represents a file of source code.
@@ skipping 49 matching lines @@
 
     var buf = new StringBuffer(
         '${filename}:${line + 1}:${column + 1}: $message');
     if (includeText) {
       buf.add('\n');
       var textLine;
       // +1 for 0-indexing, +1 again to avoid the last line of the file
       if ((line + 2) < _lineStarts.length) {
         textLine = text.substring(_lineStarts[line], _lineStarts[line+1]);
       } else {
-        textLine = text.substring(_lineStarts[line]) + '\n';
+        textLine = '${text.substring(_lineStarts[line])}\n';
       }
 
       int toColumn = Math.min(column + (end-start), textLine.length);
       if (colors.enabled) {
         buf.add(textLine.substring(0, column));
         buf.add(colors.RED_COLOR);
         buf.add(textLine.substring(column, toColumn));
         buf.add(colors.NO_COLOR);
         buf.add(textLine.substring(toColumn));
       } else {
         buf.add(textLine);
       }
 
       int i = 0;
       for (; i < column; i++) {
         buf.add(' ');
       }
 
       if (colors.enabled) buf.add(colors.RED_COLOR);
       for (; i < toColumn; i++) {
         buf.add('^');
       }
       if (colors.enabled) buf.add(colors.NO_COLOR);
     }
 
     return buf.toString();
   }
 }