Prevent experimental promotion to stable

Adds a check to the `prepare-stable` script to prevent experimental
builds from being published using stable semver versions.

scripts/release/prepare-stable.js

@@ -14,6 +14,7 @@ const printPrereleaseSummary = require('./shared-commands/print-prerelease-summa
 const testPackagingFixture = require('./shared-commands/test-packaging-fixture');
 const testTracingFixture = require('./shared-commands/test-tracing-fixture');
 const updateStableVersionNumbers = require('./prepare-stable-commands/update-stable-version-numbers');
+const theme = require('./theme');
 
 const run = async () => {
   try {
@@ -30,6 +31,13 @@ const run = async () => {
       params.version = await getLatestCanaryVersion();
     }
 
+    if (params.version.includes('experimental')) {
+      console.error(
+        theme.error`Cannot promote an experimental build to stable.`
+      );
+      process.exit(1);
+    }
+
     await checkOutPackages(params);
     await guessStableVersionNumbers(params, versionsMap);
     await confirmStableVersionNumbers(params, versionsMap);