message: fix usage of -st-
diff: @@ -236,7 +236,7 @@ export class StylableTransformer {
  } else {
  const resolvedClass = this.resolver.deepResolve(mix.ref);
  if (resolvedClass && resolvedClass.symbol && resolvedClass._kind === 'css') {
  - if ((resolvedClass.symbol as any)['-st-root']){
  + if ((resolvedClass.symbol as ClassSymbol | ElementSymbol)[valueMapping.root]) {
  let importNode = getCorrectNodeImport((mix.ref as ImportSymbol).import, (node: any) => { return node.prop === valueMapping.default })
  this.diagnostics.error(importNode, `"${importNode.value}" is a stylesheet and cannot be used as a mixin`, { word: importNode.value })
  }
  