7.7 upgrade and ScssStylesheet class not found

I'm trying to upgrade from 7.6.8 to 7.7.3, and on the fly theme compiling has stopped working:

13:02:42.002 [qtp1878814375-20] WARN  o.e.jetty.servlet.ServletHandler - Error for /VAADIN/themes/curo/styles.css
java.lang.NoClassDefFoundError: com/vaadin/sass/internal/ScssStylesheet
    at com.vaadin.server.VaadinServlet.compileScssOnTheFly(VaadinServlet.java:1106)
    at com.vaadin.server.VaadinServlet.serveOnTheFlyCompiledScss(VaadinServlet.java:1060)
    at com.vaadin.server.VaadinServlet.serveStaticResourcesInVAADIN(VaadinServlet.java:767)
    at com.vaadin.server.VaadinServlet.serveStaticResources(VaadinServlet.java:731)
    at com.vaadin.server.VaadinServlet.service(VaadinServlet.java:372)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
    at com.curocomp.client.vaadin.CuroVaadinServlet.service(CuroVaadinServlet.java:79)
    at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:812)
    at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1669)
    at org.eclipse.jetty.websocket.server.WebSocketUpgradeFilter.doFilter(WebSocketUpgradeFilter.java:224)
    at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
    at org.springframework.boot.actuate.autoconfigure.EndpointWebMvcAutoConfiguration$ApplicationContextHeaderFilter.doFilterInternal(EndpointWebMvcAutoConfiguration.java:281)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
    at org.springframework.boot.actuate.trace.WebRequestTraceFilter.doFilterInternal(WebRequestTraceFilter.java:115)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
    at org.springframework.security.web.FilterChainProxy.doFilterInternal(FilterChainProxy.java:207)
    at org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:176)
    at org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:346)
    at org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:262)
    at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
    at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
    at org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:87)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
    at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:77)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:121)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
    at org.springframework.boot.actuate.autoconfigure.MetricsFilter.doFilterInternal(MetricsFilter.java:103)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1652)
    at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:585)
    at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143)
    at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:577)
    at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:223)
    at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1127)
    at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:515)
    at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:185)
    at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1061)
    at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
    at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:97)
    at org.eclipse.jetty.server.Server.handle(Server.java:499)
    at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:311)
    at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:257)
    at org.eclipse.jetty.io.AbstractConnection$2.run(AbstractConnection.java:544)
    at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:635)
    at org.eclipse.jetty.util.thread.QueuedThreadPool$3.run(QueuedThreadPool.java:555)
    at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.ClassNotFoundException: com.vaadin.sass.internal.ScssStylesheet
    at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    ... 54 common frames omitted

Some googling reveals similar stack traces when using OSGI with on-the-fly theme compiling, but we're not using OSGI - we're using Spring boot & Maven.  Am I missing a dependency?

Thanks!
Dave

For anyone else that's looking, someone kindly pointed out to me that Vaadin has a helpful dependency for external GWT stuff:

<dependency>
            <groupId>com.vaadin.external.gwt</groupId>
            <artifactId>gwt-servlet</artifactId>
            <version>2.7.0.vaadin4</version>
        </dependency>