code: function getChildCompiler (mainCompiler) {
    const cachedChildCompiler = childCompilerCache.get(mainCompiler);
    if (cachedChildCompiler) {
      return cachedChildCompiler;
    }
    const newCompiler = new HtmlWebpackChildCompiler();
    childCompilerCache.set(mainCompiler, newCompiler);
    return newCompiler;
  }
docs: Get child compiler from cache or a new child compiler for the given mainCompilation
  
  @param {WebpackCompiler} mainCompiler