Variable !important is replaced with none when level 2 is enabled

[pawelkondraciuk]

Precheck

• Do a quick search and make sure a bug has not yet been reported;
• do a quick check if the bug still exists in the latest patch version;
• finally, be nice and have fun!

Environment

• clean-css version - 4.2.1:
• node.js version - 8.11.3:
• operating system: macOS 10.14.1

Configuration options

var CleanCSS = require('clean-css');
new CleanCSS({
  level: 2
})

Input CSS

:root {
  --border-test: 1px solid red;
}
.u-test {
  border-left: var(--border-test) !important;
}

Actual output CSS

:root{--border-test:1px solid red}.u-test{border-left:none!important}

Expected output CSS

:root{--border-test:1px solid red}.u-test{border-left:var(--border-test)!important}

Notes

Without !important flag it compiles to

:root{--border-test:1px solid red}.u-test{border-left:var(--border-test)}

[XieChuanfang]

then, how to solve it finally? @pawelkondraciuk @jakubpawlowicz I also encounter this problem, Thanks!