Properly load Dartium from the Dart Editor when possible.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1086753006

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 * Fix running VM tests against `pub serve`.
 
+* Properly load Dartium from the Dart Editor when possible.
+
 ### 0.12.0-beta.8
 
 * Add support for configuring timeouts on a test, group, and suite basis. Test

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;
   }
 }