title: Flutter web debug is working but build web shows a blank page
body: <p><code>flutter doctor</code> result</p>
  <pre><code>[√] Flutter (Channel dev, 1.21.0-1.0.pre, on Microsoft Windows [Version 10.0.19041.388], locale en-US)
  [√] Android toolchain - develop for Android devices (Android SDK version 29.0.3)
  [√] Chrome - develop for the web
  [√] Android Studio (version 4.0)
  [√] VS Code (version 1.47.2)
  [√] Connected device (3 available)
  
  • No issues found!
  </code></pre>
  <p>I was working with flutter v 1.20-7.2.pre in beta channel and encountered this problem. So I switched to dev channel.</p>
  <p>I tried running the example project (boilerplate) as well and got a blank page. The console shows some errors in main.dart.js and js_helper.dart shows the error:</p>
  <pre><code>Could not load content for org-dartlang-sdk:///lib/_internal/js_runtime/lib/js_helper.dart (HTTP error: status code 404, net::ERR_UNKNOWN_URL_SCHEME)
  </code></pre>
  <h2>To Reproduce the problem</h2>
  <p>Create the sample project in any IDE
  Run the following commands:</p>
  <pre><code>flutter channel beta
  flutter upgrade
  flutter config --enable-web
  </code></pre>
  <p>OR</p>
  <pre><code>flutter channel dev
  flutter upgrade
  flutter config --enable-web
  </code></pre>
  <p>Then, run:
  <code>flutter build web</code></p>
  <p>After this, I used python to run localhost:
  <code>python -m http.server 8080</code> in the $root_folder\build\web</p>
  <p>I found another answer which suggested hosting the build\web folder as localhost has CORS problems</p>
  <p>Also, I tried using the debug mode: <code>flutter run -d Chrome</code> and it worked!</p>
  <p><strong>Extra information:</strong> I am testing this on Chrome v 84.0.4147.89 (Official Build) (64-bit)</p>