Add comment in unittest

pkg/unittest/lib/compact_vm_config.dart

 // Copyright (c) 2013, 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.
 
 /**
  * A test configuration that generates a compact 1-line progress bar. The bar is
  * updated in-place before and after each test is executed. If all test pass,
  * you should only see a couple lines in the terminal. If a test fails, the
  * failure is shown and the progress bar continues to be updated below it.
  */
@@ skipping 83 matching lines @@
     buffer.write('+');
     buffer.write(passed);
     buffer.write(_NONE);
     if (failed != 0) buffer.write(_RED);
     buffer.write(' -');
     buffer.write(failed);
     if (failed != 0) buffer.write(_NONE);
     buffer.write(': ');
     buffer.write(color);
 
+    // Ensure the line fits under MAX_LINE. [buffer] includes the color escape
+    // sequences too. Because these sequences are not visible characters, we
+    // make sure they are not counted towards the limit.
     int nonVisible = _nonVisiblePrefix + color.length  +
         (failed != 0 ? (_RED.length + _NONE.length) : 0);
     int len = buffer.length - nonVisible;
     var mx = MAX_LINE - len;
     buffer.write(_snippet(message, MAX_LINE - len));
     buffer.write(_NONE);
 
     // Pad the rest of the line so that it looks erased.
     len = buffer.length - nonVisible - _NONE.length;
     if (len > _lastLength) {
@@ skipping 50 matching lines @@
       res = res.substring(firstSpace);
     }
     return '...$res';
   }
 }
 
 void useCompactVMConfiguration() {
   if (config != null) return;
   configure(new CompactVMConfiguration());
 }