HTMLSelectElement: fix selectedOptions when selectedIndex < 0

Closes https://github.com/dart-lang/sdk/pull/54618

Also adds a comment to go.sh mentioning vpython3 comes with
depot_tools.

GitOrigin-RevId: 5e057703067c24d9d41bac75b6251c8f3a2fddd3
Change-Id: I937d8618c24602b4c0844d8a52ff00095836bca8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/346220
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Commit-Queue: Srujan Gaddam <srujzs@google.com>

sdk/lib/html/dart2js/html_dart2js.dart

@@ -27715,7 +27715,8 @@ class SelectElement extends HtmlElement {
       var options = this.options.where((o) => o.selected).toList();
       return new UnmodifiableListView(options);
     } else {
-      return [this.options[this.selectedIndex!]];
+      var i = this.selectedIndex!;
+      return i < 0 ? [] : [this.options[i]];
     }
   }
 }

tools/dom/scripts/go.sh

@@ -35,6 +35,9 @@ if [[ "$1" != "" ]] ; then
   fi
 fi
 
+# vpython3 is available through depot_tools. See
+# https://github.com/dart-lang/sdk/wiki/Building for instructions on installing
+# depot_tools.
 reset && \
 vpython3 ./dartdomgenerator.py --systems="$SYSTEMS" --logging=40 --update-dom-metadata --gen-interop "$ARG_OPTION"
 

tools/dom/templates/html/impl/impl_HTMLSelectElement.darttemplate

@@ -20,7 +20,8 @@ $!MEMBERS
       var options = this.options.where((o) => o.selected).toList();
       return new UnmodifiableListView(options);
     } else {
-      return [this.options[this.selectedIndex$NULLASSERT]];
+      var i = this.selectedIndex$NULLASSERT;
+      return i < 0 ? [] : [this.options[i]];
     }
   }
 }