Properly load Dartium from the Dart Editor when possible.

lib/src/runner/browser/dartium.dart

Index: lib/src/runner/browser/dartium.dart
diff --git a/lib/src/runner/browser/dartium.dart b/lib/src/runner/browser/dartium.dart
index ecb3289c9ae62ac4abd251d8fda367ab654b3381..f8d81513e00bad2cd386b5ad018676fbc8d323b8 100644
--- a/lib/src/runner/browser/dartium.dart
+++ b/lib/src/runner/browser/dartium.dart
@@ -96,7 +96,7 @@ class Dartium implements Browser {
       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) {
@@ -106,14 +106,14 @@ class Dartium implements Browser {
       }
 
       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;
   }
 }