Migrate pub away from throwing strings.

utils/pub/utils.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.
 
 /// Generic utility functions. Stuff that should possibly be in core.
 library utils;
 
 import 'dart:async';
 import 'dart:crypto';
+import 'dart:io';
 import 'dart:isolate';
 import 'dart:uri';
 
 /// A pair of values.
 class Pair<E, F> {
   E first;
   F last;
 
   Pair(this.first, this.last);
 
@@ skipping 331 matching lines @@
         .then((resolved) => new Pair(key, resolved)));
   });
   return Future.wait(pairs).then((resolvedPairs) {
     var map = {};
     for (var pair in resolvedPairs) {
       map[pair.first] = pair.last;
     }
     return map;
   });
 }
+
+/// An exception class for exceptions that are intended to be seen by the user.
+/// These exceptions won't have any debugging information printed when they're
+/// thrown.
+class UserFacingException implements Exception {
+  final String message;
+
+  UserFacingException(this.message);
+}
+
+/// Throw a [UserFacingException] with [message].
+void fail(String message) {
+  throw new UserFacingException(message);
+}
+
+/// Returns whether [error] is a user-facing error object. This includes both
+/// [UserFacingException] and any dart:io errors.
+bool isUserFacingException(error) {
+  return error is UserFacingException ||
+    // TODO(nweiz): clean up this branch when issue 9955 is fixed.
+    error is DirectoryIOException ||
+    error is FileIOException ||
+    error is HttpException ||
+    error is HttpParserException ||
+    error is LinkIOException ||
+    error is MimeParserException ||
+    error is OSError ||
+    error is ProcessException ||
+    error is SocketIOException ||
+    error is WebSocketException;
+}