Fix all strong-mode errors and warnings.

lib/src/runner/browser/phantom_js.dart

Index: lib/src/runner/browser/phantom_js.dart
diff --git a/lib/src/runner/browser/phantom_js.dart b/lib/src/runner/browser/phantom_js.dart
index 5ca5cd136c2ca2e21fd9539b16d0764ab45a4535..a3097e049bab66912e90a7ecf5a7e1c2ce2f62f7 100644
--- a/lib/src/runner/browser/phantom_js.dart
+++ b/lib/src/runner/browser/phantom_js.dart
@@ -40,7 +40,7 @@ class PhantomJS extends Browser {
   final Future<Uri> remoteDebuggerUrl;
 
   factory PhantomJS(url, {String executable, bool debug: false}) {
-    var remoteDebuggerCompleter = new Completer.sync();
+    var remoteDebuggerCompleter = new Completer<Uri>.sync();
     return new PhantomJS._(() async {
       if (executable == null) {
         executable = Platform.isWindows ? "phantomjs.exe" : "phantomjs";
@@ -52,7 +52,7 @@ class PhantomJS extends Browser {
 
       var port = debug ? await getUnsafeUnusedPort() : null;
 
-      var args = [];
+      var args = <String>[];
       if (debug) {
         args.addAll([
           "--remote-debugger-port=$port",