Eagerly close browser tests.

lib/src/runner.dart

Index: lib/src/runner.dart
diff --git a/lib/src/runner.dart b/lib/src/runner.dart
index 95caaf6810f46f018d92842668b62389c26b81d0..517e3a09f8c5b2597993b819f249fd93b38e6640 100644
--- a/lib/src/runner.dart
+++ b/lib/src/runner.dart
@@ -166,7 +166,14 @@ class Runner {
 
     // Make sure we close the engine *before* the loader. Otherwise,
     // LoadSuites provided by the loader may get into bad states.
-    await _engine.close();
+    //
+    // We close the loader's browsers while we're closing the engine because
+    // browser tests don't store any state we care about and we want them to
+    // shut down without waiting for their tear-downs.
+    await Future.wait([
+      _loader.closeBrowsers(),
+      _engine.close()
+    ]);
     if (timer != null) timer.cancel();
     await _loader.close();
   });