Properly load Dartium from the Dart Editor when possible.

lib/src/runner/browser/dartium.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.dartium;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
@@ skipping 78 matching lines @@
     return Platform.isWindows ? "dartium.exe" : "dartium";
   }
 
   String _executableInEditor() {
     var dir = p.dirname(sdkDir);
 
     if (Platform.isWindows) {
       if (!new File(p.join(dir, "DartEditor.exe")).existsSync()) return null;
 
       var dartium = p.join(dir, "chromium\\chrome.exe");
-      return new File(dartium).existsSync() ? null : dartium;
+      return new File(dartium).existsSync() ? dartium : null;
     }
 
     if (Platform.isMacOS) {
       if (!new File(p.join(dir, "DartEditor.app/Contents/MacOS/DartEditor"))
           .existsSync()) {
         return null;
       }
 
       var dartium = p.join(
-          dir, "chromium/Chromium.app/Contents/MacOs/Chromium");
-      return new File(dartium).existsSync() ? null : dartium;
+          dir, "chromium/Chromium.app/Contents/MacOS/Chromium");
+      return new File(dartium).existsSync() ? dartium : null;
     }
 
     assert(Platform.isLinux);
     if (!new File(p.join(dir, "DartEditor")).existsSync()) return null;
 
     var dartium = p.join(dir, "chromium", "chrome");
-    return new File(dartium).existsSync() ? null : dartium;
+    return new File(dartium).existsSync() ? dartium : null;
   }
 }