Bootstrap 4.3.1升级：没有名称为font-size的混合文件

Question

我有一个正在运行Bootstrap 4beta的Angular项目。我通过命令行npm install bootstrap --save升级到了最新的Bootstrap 4.3.1。我也做了npm install。但是，当我执行ng serve时，出现以下错误：没有名称为font-size的混合。有想法该怎么解决这个吗？

以下是我的_mixins.scss文件

// Toggles
//
// Used in conjunction with global variables to enable certain theme features.

// Vendor
@import "vendor/rfs";

// Deprecate
@import "mixins/deprecate";

// Utilities
@import "mixins/breakpoints";
@import "mixins/hover";
@import "mixins/image";
@import "mixins/badge";
@import "mixins/resize";
@import "mixins/screen-reader";
@import "mixins/size";
@import "mixins/reset-text";
@import "mixins/text-emphasis";
@import "mixins/text-hide";
@import "mixins/text-truncate";
@import "mixins/visibility";

// // Components
@import "mixins/alert";
@import "mixins/buttons";
@import "mixins/caret";
@import "mixins/pagination";
@import "mixins/lists";
@import "mixins/list-group";
@import "mixins/nav-divider";
@import "mixins/forms";
@import "mixins/table-row";

// // Skins
@import "mixins/background-variant";
@import "mixins/border-radius";
@import "mixins/box-shadow";
@import "mixins/gradients";
@import "mixins/transition";

// // Layout
@import "mixins/clearfix";
@import "mixins/grid-framework";
@import "mixins/grid";
@import "mixins/float";