pkg/test: Resolve a number of analyzer warnings

lib/src/runner/browser/content_shell.dart

Index: lib/src/runner/browser/content_shell.dart
diff --git a/lib/src/runner/browser/content_shell.dart b/lib/src/runner/browser/content_shell.dart
index 35908dcf8532779d596f5741a3d4d6920227f8fe..c3002404b523018bbb59a86027f573efe68f78bb 100644
--- a/lib/src/runner/browser/content_shell.dart
+++ b/lib/src/runner/browser/content_shell.dart
@@ -105,7 +105,7 @@ class ContentShell extends Browser {
       var client = new HttpClient();
       var request = await client.getUrl(base.resolve("/json/list"));
       var response = await request.close();
-      var json = await JSON.fuse(UTF8).decoder.bind(response).single;
+      var json = await JSON.fuse(UTF8).decoder.bind(response).single as List;

[nweiz, 2016/06/20 20:51:36]

Why doesn't this complain about calling operator [] on the result of json.first?

[kevmoo, 2016/06/20 20:57:49]

You get Object out of the fused decoder, which is not a List.

Each item in `List` is dynamic, though – so of course it has an index operator

       return base.resolve(json.first["devtoolsFrontendUrl"]);
     } catch (_) {
       // If we fail to talk to the remote debugger protocol, give up and return