Add a convenience test task for vscode (#9570)

packages/flutter/.gitignore

@@ -3,3 +3,4 @@
 pubspec.lock
 doc/api/
 coverage/
+.vscode

packages/flutter/.vscode/tasks.json

+{
+  "version": "0.1.0",
+  "command": "flutter",
+  "args": [],
+  "showOutput": "always",
+  "echoCommand": true,
+  "tasks": [
+    {
+      // Assign key binding to workbench.action.tasks.test to quickly run 
+      // the currently open test. 
+      "taskName": "test",
+      "isTestCommand": true,
+      "isShellCommand": true,
+      "args": ["${file}"]
+    }
+  ]
+}