More gracefully handle browser errors.

lib/src/runner/browser/server.dart

Index: lib/src/runner/browser/server.dart
diff --git a/lib/src/runner/browser/server.dart b/lib/src/runner/browser/server.dart
index fb0e55abddf6f16b6d0569b6715785ac54854bd8..17862ebcb7fa4b844b4d49782f8f2a60442c7361 100644
--- a/lib/src/runner/browser/server.dart
+++ b/lib/src/runner/browser/server.dart
@@ -266,7 +266,7 @@ void main() {
 
       // TODO(nweiz): Don't start the browser until all the suites are compiled.
       return _browserManagerFor(browser).then((browserManager) {
-        if (_closed) return null;
+        if (_closed || browserManager == null) return null;
         return browserManager.loadSuite(path, suiteUrl, metadata);
       }).then((suite) {
         if (_closed) return null;