Add a test runner executable.

lib/src/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.load_exception;
+
+import 'dart:isolate';
+
+import 'package:path/path.dart' as p;
+
+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 "$path":');
+
+    var innerString = getErrorMessage(innerError);
+    if (innerError is IsolateSpawnException) {
+      // If this is a parse error, get rid of the noisy preamble.
+      innerString = innerString
+          .replaceFirst("'${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();
+    }
+
+    buffer.write(innerString.contains("\n") ? "\n" : " ");
+    buffer.write(innerString);
+    return buffer.toString();
+  }
+}