Fixing windows build (#849)

Co-authored-by: Ingvar Stepanyan <rreverser@google.com>

lib/css-plugin.js

@@ -18,6 +18,8 @@ import {
   resolve as resolvePath,
   dirname,
   normalize as nomalizePath,
+  sep as pathSep,
+  posix,
 } from 'path';
 
 import postcss from 'postcss';
@@ -172,14 +174,15 @@ export default function (resolveFileUrl) {
         return `export default ${cssStr};`;
       }
       if (id.startsWith(addPrefix)) {
-        const path = id.slice(addPrefix.length);
+        const path = nomalizePath(id.slice(addPrefix.length));
         return (
-          `import css from 'css:${path}';\n` +
+          `import css from ${JSON.stringify('css:' + path)};\n` +
           `import appendCss from '${appendCssModule}';\n` +
           `appendCss(css);\n`
         );
       }
       if (id.endsWith(moduleSuffix)) {
+        const path = nomalizePath(id.slice(0, -moduleSuffix.length));
         if (!pathToResult.has(id)) {
           throw Error(`Cannot find ${id} in pathToResult`);
         }