private void expect(com.fasterxml.jackson.core.JsonToken expected) {
    if (peek != expected) {
      throw new IllegalStateException("Expected " + expected + " but was " + peek);
    }
  }
{@link JsonReader} implementation backed by Jackson's {@link JsonParser}.
Provides measurable JSON parsing improvements over Gson's native implementation.
Error reporting might differ, however.
gson/src/org/immutables/gson/stream/JsonParserReader.java