Error during gulp build

[jjsaunier]

Error in plugin 'gulp-sass'
Message:
    client/styles/plugins/_angular-bootstrap.scss
Error: ".ui-datepicker .btn-group .btn" failed to @extend ".btn-flat".
       The selector ".btn-flat" was not found.
       Use "@extend .btn-flat !optional" if the extend should be able to fail.
        on line 69 of client/styles/plugins/_angular-bootstrap.scss
>>         @extend .btn-flat;
   --------^

Details:
    formatted: Error: ".ui-datepicker .btn-group .btn" failed to @extend ".btn-flat".
       The selector ".btn-flat" was not found.
       Use "@extend .btn-flat !optional" if the extend should be able to fail.
        on line 69 of client/styles/plugins/_angular-bootstrap.scss
>>         @extend .btn-flat;
   --------^

    column: 9
    line: 69
    file: /app/client/styles/plugins/_angular-bootstrap.scss
    status: 1
    messageFormatted: client/styles/plugins/_angular-bootstrap.scss
Error: ".ui-datepicker .btn-group .btn" failed to @extend ".btn-flat".
       The selector ".btn-flat" was not found.
       Use "@extend .btn-flat !optional" if the extend should be able to fail.
        on line 69 of client/styles/plugins/_angular-bootstrap.scss
>>         @extend .btn-flat;
   --------^

    messageOriginal: ".ui-datepicker .btn-group .btn" failed to @extend ".btn-flat".
The selector ".btn-flat" was not found.
Use "@extend .btn-flat !optional" if the extend should be able to fail.
    relativePath: client/styles/plugins/_angular-bootstrap.scss

So the viewer in browser is broken