Respect top-level @TestOn declarations.

lib/src/runner/load_exception.dart

 // Copyright (c) 2015, 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 unittest.runner.load_exception;
 
 import 'dart:isolate';
 
 import 'package:path/path.dart' as p;
+import 'package:source_span/source_span.dart';
 
 import '../utils.dart';
 
 class LoadException implements Exception {
   final String path;
 
   final innerError;
 
   LoadException(this.path, this.innerError);
 
-  String toString() {
-    var buffer = new StringBuffer('Failed to load "$path":');
+  String toString({bool color: false}) {
+    var buffer = new StringBuffer();
+    if (color) buffer.write('\u001b[31m'); // red
+    buffer.write('Failed to load "$path":');
+    if (color) buffer.write('\u001b[0m'); // no color
 
     var innerString = getErrorMessage(innerError);
     if (innerError is IsolateSpawnException) {
       // If this is a parse error, get rid of the noisy preamble.
       innerString = innerString
           .replaceFirst("'${p.toUri(p.absolute(path))}': error: ", "");
 
       // If this is a file system error, get rid of both the preamble and the
       // useless stack trace.
       innerString = innerString.replaceFirst(
           "Unhandled exception:\n"
           "Uncaught Error: Load Error: FileSystemException: ",
           "");
       innerString = innerString.split("Stack Trace:\n").first.trim();
+    } if (innerError is SourceSpanException) {
+      innerString = innerError.toString(color: color)
+          .replaceFirst(" of $path", "");
     }
 
     buffer.write(innerString.contains("\n") ? "\n" : " ");
     buffer.write(innerString);
     return buffer.toString();
   }
 }