Move a bunch of src/ files into subdirectories.

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();
-  }
-}