More gracefully handle browser errors.

lib/src/runner/browser/firefox.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 test.runner.browser.firefox;
 
 import 'dart:async';
+import 'dart:convert';
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
+import 'package:stack_trace/stack_trace.dart';
 
 import '../../util/io.dart';
+import '../../utils.dart';
+import '../application_exception.dart';
 import 'browser.dart';
 
 final _preferences = '''
 user_pref("browser.shell.checkDefaultBrowser", false);
 user_pref("dom.disable_open_during_load", false);
 user_pref("dom.max_script_run_time", 0);
 ''';
 
 /// A class for running an instance of Firefox.
 ///
 /// Most of the communication with the browser is expected to happen via HTTP,
 /// so this exposes a bare-bones API. The browser starts as soon as the class is
 /// constructed, and is killed when [close] is called.
 ///
 /// Any errors starting or running the process are reported through [onExit].
 class Firefox implements Browser {
   /// The underlying process.
   Process _process;
 
-  /// The temporary directory used as the browser's user data dir.
-  ///
-  /// A new data dir is created for each run to ensure that they're
-  /// well-isolated.
-  String _dir;
-
   Future get onExit => _onExitCompleter.future;
   final _onExitCompleter = new Completer();
 
   /// A future that completes when the browser process has started.
   ///
   /// This is used to ensure that [close] works regardless of when it's called.
   Future get _onProcessStarted => _onProcessStartedCompleter.future;
   final _onProcessStartedCompleter = new Completer();
 
   /// Starts a new instance of Firefox open to the given [url], which may be a
   /// [Uri] or a [String].
   ///
   /// If [executable] is passed, it's used as the Firefox executable. Otherwise
   /// the default executable name for the current OS will be used.
   Firefox(url, {String executable}) {
     if (executable == null) executable = _defaultExecutable();
 
     // Don't return a Future here because there's no need for the caller to wait
     // for the process to actually start. They should just wait for the HTTP
     // request instead.
     withTempDir((dir) {
-      _dir = dir;
-
       new File(p.join(dir, 'prefs.js')).writeAsStringSync(_preferences);
 
       return Process.start(executable, [
         "--profile",
-        "$_dir",
+        "$dir",
         url.toString(),
         "--no-remote"
       ], environment: {
         "MOZ_CRASHREPORTER_DISABLE": "1"
       }).then((process) {
         _process = process;
         _onProcessStartedCompleter.complete();
 
         // TODO(nweiz): the browser's standard output is almost always useless
         // noise, but we should allow the user to opt in to seeing it.
         return _process.exitCode;
       });
     }).then((exitCode) {
-      if (exitCode != 0) throw "Firefox failed with exit code $exitCode.";
-    }).then(_onExitCompleter.complete)
-        .catchError(_onExitCompleter.completeError);
+      if (exitCode == 0) return null;
+
+      return UTF8.decodeStream(_process.stderr).then((error) {
+        throw new ApplicationException(
+            "Firefox failed with exit code $exitCode:\n$error");
+      });
+    }).then(_onExitCompleter.complete).catchError((error, stackTrace) {
+      if (stackTrace == null) stackTrace = new Trace.current();
+      _onExitCompleter.completeError(
+          new ApplicationException(
+              "Failed to start Firefox: ${getErrorMessage(error)}."),
+          stackTrace);
+    });
   }
 
   Future close() {
     _onProcessStarted.then((_) => _process.kill());
 
     // Swallow exceptions. The user should explicitly use [onExit] for these.
     return onExit.catchError((_) {});
   }
 
   /// Return the default executable for the current operating system.
@@ skipping 16 matching lines @@
 
       var path = p.join(prefix, suffix);
       if (new File(p.join(prefix, suffix)).existsSync()) return path;
     }
 
     // Fall back on looking it up on the path. This probably won't work, but at
     // least it will fail with a useful error message.
     return "firefox.exe";
   }
 }