Fixed incorrect import of collection request

Author: VShingala

codegens/dart-http/test/unit/convert.test.js

@@ -1,6 +1,6 @@
 var convert = require('../../index').convert,
   expect = require('chai').expect,
-  sdk = require('postman-collection');
+  { Request } = require('postman-collection/lib/collection/request');
 
 // Disable check with expected snippets as we now have proper newman tests
 describe('Dart Converter', function () {