ci(typescript): add typescript_next build

Install typescript@next before build.js and test.typings.
Restore the regular version before travis caches node_modules/.

Fixes #6368

scripts/ci/build_and_test.sh

@@ -19,6 +19,11 @@ elif [ "$MODE" = "lint" ]; then
 elif [ "$MODE" = "build_only" ]; then
   ${SCRIPT_DIR}/build_js.sh
   ${SCRIPT_DIR}/build_dart.sh
+elif [ "$MODE" = "typescript_next" ]; then
+   # Ignore complaints about unsatisfied peer deps
+   npm install typescript@next || true
+   ${SCRIPT_DIR}/build_js.sh
+   ./node_modules/.bin/gulp test.typings
 elif [ "$MODE" = "payload" ]; then
   source ${SCRIPT_DIR}/env_dart.sh
   ./node_modules/.bin/gulp test.payload.dart/ci