code: function () {
        var sheets = this.sheets.filter(function(s) {
          return !s.hasAttribute(SCOPE_ATTR);
        });
        var content = this.templateContent();
        if (content) {
          var cssText = '';
          sheets.forEach(function(sheet) {
            cssText += cssTextFromSheet(sheet) + '\n';
          });
          if (cssText) {
            var style = createStyleElement(cssText, this.ownerDocument);
            content.insertBefore(style, content.firstChild);
          }
        }
      }
docs: Takes external stylesheets loaded in an <element> element and moves
  their content into a <style> element inside the <element>'s template.
  The sheet is then removed from the <element>. This is done only so
  that if the element is loaded in the main document, the sheet does
  not become active.
  Note, ignores sheets with the attribute 'polymer-scope'.
  @param elementElement The <element> element to style.