fix(@angular/cli): ensure stylesheet relative tilde path conversion

Author: clydin

packages/@angular/cli/models/webpack-configs/styles.ts

@@ -44,11 +44,14 @@ export function getStylesConfig(wco: WebpackConfigOptions) {
   const baseHref = wco.buildOptions.baseHref || '';
   const deployUrl = wco.buildOptions.deployUrl || '';
 
-  const postcssPluginCreator = function() {
+  const postcssPluginCreator = function(loader: any) {
     return [
       postcssUrl({
         filter: ({ url }: { url: string }) => url.startsWith('~'),
-        url: ({ url }: { url: string }) => path.join(projectRoot, 'node_modules', url.substr(1)),
+        url: ({ url }: { url: string }) => {
+          const fullPath = path.join(projectRoot, 'node_modules', url.substr(1));
+          return path.relative(loader.context, fullPath);
+        }
       }),
       postcssUrl([
         {