diff --git a/doc/tutorials/chapter_6/answers/exercise_2_n_bit_subtractor.dart b/doc/tutorials/chapter_6/answers/exercise_2_n_bit_subtractor.dart
index 5765f08bf..18efb5a2d 100644
--- a/doc/tutorials/chapter_6/answers/exercise_2_n_bit_subtractor.dart
+++ b/doc/tutorials/chapter_6/answers/exercise_2_n_bit_subtractor.dart
@@ -7,7 +7,6 @@ import '../../chapter_3/answers/helper.dart';
 import '../../chapter_5/answers/full_subtractor.dart';
 
 class FullSubtractorComb extends FullSubtractor {
-  @override
   FullSubtractorComb(super.a, super.b, super.borrowIn) {
     // Declare input and output
     final a = input('a');
diff --git a/tool/gh_codespaces/install_dart.sh b/tool/gh_codespaces/install_dart.sh
index abbe39a0c..d0bfdfe91 100755
--- a/tool/gh_codespaces/install_dart.sh
+++ b/tool/gh_codespaces/install_dart.sh
@@ -8,24 +8,93 @@
 #
 # 2023 February 5
 # Author: Chykon
+#
+# 2026 June 21
+# Updated to add fallback logic for fetching the latest Dart repository key from Google if the locally cached key fails verification (e.g. due to key rotation).
+# Author: Desmond A. Kirkpatrick <desmond.a.kirkpatrick@intel.com>
 
 set -euo pipefail
 
-# Add Dart repository key.
+declare -r cached_pubkey_file="$(dirname "${BASH_SOURCE[0]}")/pubkeys/dart.pub"
+declare -r keyring_file='/usr/share/keyrings/dart.gpg'
+declare -r dart_repository_file='/etc/apt/sources.list.d/dart_stable.list'
+declare -r dart_repository_url='https://storage.googleapis.com/download.dartlang.org/linux/debian'
+declare -r google_signing_key_url='https://dl-ssl.google.com/linux/linux_signing_key.pub'
 
-declare -r input_pubkey_file='tool/gh_codespaces/pubkeys/dart.pub'
-declare -r output_pubkey_file='/usr/share/keyrings/dart.gpg'
+sudo apt-get update
+sudo apt-get install -y wget gpg apt-transport-https
 
-sudo gpg --output ${output_pubkey_file} --dearmor ${input_pubkey_file}
+sudo mkdir -p /usr/share/keyrings
 
 # Add Dart repository.
 
-declare -r dart_repository_url='https://storage.googleapis.com/download.dartlang.org/linux/debian'
-declare -r dart_repository_file='/etc/apt/sources.list.d/dart.list'
+echo "deb [signed-by=${keyring_file}] ${dart_repository_url} stable main" \
+  | sudo tee "${dart_repository_file}"
+
+# Install the repository key from the locally cached, ASCII-armored public key.
+install_key_from_file() {
+  sudo gpg --yes --output "${keyring_file}" --dearmor "${1}"
+}
+
+# Install the repository key by fetching the latest key from Google.
+install_key_from_google() {
+  wget -qO- "${google_signing_key_url}" \
+    | gpg --dearmor \
+    | sudo tee "${keyring_file}" >/dev/null
+}
+
+# Emit a prominent warning that stands out in CI logs (and as a GitHub Actions
+# annotation when available) without failing the build.
+warn_loudly() {
+  local message="${1}"
+  {
+    echo ''
+    echo '################################################################################'
+    echo '## install_dart WARNING'
+    echo "## ${message}"
+    echo '################################################################################'
+    echo ''
+  } >&2
+  # Surface a GitHub Actions warning annotation (non-fatal) when running in CI.
+  if [[ -n "${GITHUB_ACTIONS:-}" ]]; then
+    echo "::warning title=install_dart cached key bypassed::${message}"
+  fi
+}
+
+# Verify that the installed keyring can authenticate the Dart repository by
+# refreshing only the Dart sources list and checking for signature/key errors.
+dart_repository_verified() {
+  local update_log
+  if ! update_log=$(sudo apt-get update \
+        -o Dir::Etc::sourcelist="${dart_repository_file}" \
+        -o Dir::Etc::sourceparts="-" \
+        -o APT::Get::List-Cleanup="0" 2>&1); then
+    return 1
+  fi
+  if echo "${update_log}" \
+      | grep -Eiq 'NO_PUBKEY|EXPKEYSIG|REVKEYSIG|BADSIG|not signed|could.?n.?t be verified'; then
+    return 1
+  fi
+  return 0
+}
+
+# Prefer the locally cached key. If it can no longer authenticate the repository
+# (e.g. the key has been rotated), fall back to fetching the latest key from
+# Google so the install can still proceed.
+install_key_from_file "${cached_pubkey_file}"
 
-echo "deb [signed-by=${output_pubkey_file}] ${dart_repository_url} stable main" | sudo tee ${dart_repository_file}
+if dart_repository_verified; then
+  echo 'install_dart: using locally cached Dart repository key.'
+else
+  install_key_from_google
+  if ! dart_repository_verified; then
+    echo 'install_dart: Dart repository key verification failed even after fetching the latest key from Google.' >&2
+    exit 1
+  fi
+  warn_loudly "Cached Dart repository key (${cached_pubkey_file}) failed verification and was bypassed; installed using the latest key fetched from Google. Please refresh the cached key."
+fi
 
 # Install Dart.
 
 sudo apt-get update
-sudo apt-get install dart
+sudo apt-get install -y dart
diff --git a/tool/gh_codespaces/pubkeys/dart.pub b/tool/gh_codespaces/pubkeys/dart.pub
index 0366239cb..839f8a235 100644
--- a/tool/gh_codespaces/pubkeys/dart.pub
+++ b/tool/gh_codespaces/pubkeys/dart.pub
@@ -1,35 +1,4 @@
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