Fix Dart 2 parse bugs in the VM (#810)

I'm not sure why the analyzer doesn't catch these, they appear to be
parse time bugs not runtime bugs when executing in the vm with
--preview-dart-2

lib/src/runner/browser/chrome.dart

@@ -79,7 +79,7 @@ class Chrome extends Browser {
       };
 
       if (!debug) return tryPort();
-      return getUnusedPort<Future<Process>>(tryPort);
+      return getUnusedPort<Process>(tryPort);
     }, remoteDebuggerCompleter.future);
   }
 

lib/src/runner/browser/dartium.dart

@@ -117,7 +117,7 @@ class Dartium extends Browser {
       };
 
       if (!debug) return tryPort();
-      return getUnusedPort<Future<Process>>(tryPort);
+      return getUnusedPort<Process>(tryPort);
     }, observatoryCompleter.future, remoteDebuggerCompleter.future);
   }