void visitAbstractType(final int frameIndex, final int abstractType) {
        currentFrame[frameIndex] = abstractType;
    }
Sets an abstract type in {@link #currentFrame}.

@param frameIndex   the index of the element to be set in {@link #currentFrame}.
@param abstractType an abstract type.
core/src/main/java/com/alibaba/fastjson2/internal/asm/MethodWriter.java