title: TypeError while gulp typescript
body: <p>I am getting the following error when I execute my Typescript transpilation:</p>
  
  <pre><code>TypeError: file.isSymbolic is not a function
      at DestroyableTransform.normalize [as _transform] (D:\project\node_modules\vinyl-fs\lib\dest\prepare.js:31:15)
      at DestroyableTransform.Transform._read (D:\project\node_modules\vinyl-fs\node_modules\readable-stream\lib\_stream_transform.js:182:10)
      at DestroyableTransform.Transform._write (D:\project\node_modules\vinyl-fs\node_modules\readable-stream\lib\_stream_transform.js:170:83)
      at doWrite (D:\project\node_modules\vinyl-fs\node_modules\readable-stream\lib\_stream_writable.js:406:64)
      at writeOrBuffer (D:\project\node_modules\vinyl-fs\node_modules\readable-stream\lib\_stream_writable.js:395:5)
      at DestroyableTransform.Writable.write (D:\project\node_modules\vinyl-fs\node_modules\readable-stream\lib\_stream_writable.js:322:11)
      at Pumpify.Duplexify._write (D:\project\node_modules\pumpify\node_modules\duplexify\index.js:201:22)
      at doWrite (D:\project\node_modules\pumpify\node_modules\readable-stream\lib\_stream_writable.js:406:64)
      at writeOrBuffer (D:\project\node_modules\pumpify\node_modules\readable-stream\lib\_stream_writable.js:395:5)
      at Pumpify.Writable.write (D:\project\node_modules\pumpify\node_modules\readable-stream\lib\_stream_writable.js:322:11)
  </code></pre>
  
  <p>My gulp task looks like this: </p>
  
  <pre><code>gulp.task('tsc', function () {
      let tsResult =  gulp.src(srcPaths.tsFiles)
          .pipe(tsProject()); // tsProject created previously
  
      return tsResult.js
         .pipe(gulp.dest(buildPath)); // -&gt; the gulp.dest(..) command causes the error
  });
  </code></pre>
  
  <p>package.json devDependencies (only gulp and ts-relevant dependencies) looks like this:</p>
  
  <pre><code>{
    "devDependencies": {
      "gulp": "github:gulpjs/gulp#4.0",
      "gulp-sourcemaps": "^2.6.1",
      "gulp-tslint": "^8.1.2",
      "gulp-typescript": "^3.2.3",
      "tslint": "^5.8.0",
      "typescript": "^2.5.0",
    }
  }
  </code></pre>
  
  <p>Node version: 8.9.3 &amp; npm version: 5.6.0</p>
  
  <p>Anyone has an idea what to do?</p>