Finalize cleanup of features relying on old DartPad (dart-lang#5552)

- Introduces how to use the exercises on the async await and iterables
codelab pages
- Updates some mentions of already removed DartPad features (old
channel, HTML, etc)
- Replaces "Try Dart" embedded DartPad that's on homepage with simple
embed
- For now, it can rely on the samples present in the DartPad UI, which
we plan to add to
  - Can revisit custom examples in the future.
- A bit of a loss, but the current ones don't show Dart's newest, most
powerful futures anyway
  
Closes dart-lang#5382
Closes dart-lang#2578

Author: parlough

analysis_options.yaml

@@ -1,2 +1,2 @@
 analyzer:
-  exclude: [_site, site-shared, src, tmp, tool/dartpad_picker, tool/get_dart]
+  exclude: [_site, site-shared, src, tmp, tool/get_dart]

src/_sass/_dash.scss

@@ -363,6 +363,7 @@ $dash-max-width: 1330px;
     iframe {
       flex: 1;
       border: 1px solid $dash-dartpad-border;
+      resize: none;
     }
   }
 }