Reduce home page load time by lazy loading DartPad iframe (#5272)

[Lazy loads](https://web.dev/articles/iframe-lazy-loading) the DartPad
iframe on the (bottom of the homepage) since it is almost always
offscreen on first load and does not need to delay other parts of site
loading. The browsers that respect this can choose to load it based on
certain heuristics, so it may not make a difference, but across a few
runs I saw the largest contentful paint time in lighthouse reduce by
1-5%.

src/assets/dash/js/dartpad_picker_main.dart.js

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src/index.html

@@ -429,7 +429,7 @@ <h3>Want more practice? <a href="/language">Learn the language</a>,
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                 <div class="embedded-video-wrapper">
                     <div id="video-player"></div>
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+<!--                    <iframe id="video-player-iframe" class="embedded-video-wrapper__frame" width="auto" height="auto" src="https://youtube.com/embed/5F-6n_2XWR8?cc_lang_pref=en&cc_load_policy=1&enablejsapi=1" loading="lazy" frameborder="0" allow="autoplay; encrypted-media" allowfullscreen></iframe>-->
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tool/dartpad_picker/lib/dartpad_picker.dart

@@ -54,7 +54,9 @@ class DartPadPicker {
 
   void _initDartPad() {
     _iFrameElement = IFrameElement()
-      ..src = iFrameSrc(theme: 'dark', mode: 'dart');
+      ..src = iFrameSrc(theme: 'dark', mode: 'dart')
+      ..attributes['loading'] = 'lazy';
+
     final id = frameId;
     if (id != null) {
       _iFrameElement.id = id;

tool/dartpad_picker/pubspec.yaml

@@ -8,4 +8,4 @@ dev_dependencies:
   analysis_defaults:
     path: ../../site-shared/packages/analysis_defaults
   build_runner: ^2.4.6
-  build_web_compilers: ^4.0.5
+  build_web_compilers: ^4.0.6