feat(typescript): update to 1.9 nightly.

To workaround https://github.com/Microsoft/TypeScript/issues/7573
we must remove the readonly keyword from generated .d.ts files.
This solution will not scale, but will probably buy enough time to require our users move to a 2.0 beta.

Closes #8003

scripts/ci/build_and_test.sh

@@ -23,7 +23,10 @@ elif [ "$MODE" = "typescript_next" ]; then
    # Ignore complaints about unsatisfied peer deps
    npm install typescript@next || true
    ${SCRIPT_DIR}/build_js.sh
-   ./node_modules/.bin/gulp test.typings
+   # Run typings test using the GA release of TypeScript
+   # This ensures that users aren't forced onto beta/nightly
+   npm install typescript@1.8.9
+   ./node_modules/.bin/gulp \!test.typings
 elif [ "$MODE" = "payload" ]; then
   source ${SCRIPT_DIR}/env_dart.sh
   ./node_modules/.bin/gulp test.payload.dart/ci