fix(@angular/cli): Bail out if output path is the root folder (angular#4490)

`ng serve` --output-path=. would wipe out the entire root folder.

Author: catull

packages/@angular/cli/tasks/serve.ts

@@ -24,6 +24,9 @@ export default Task.extend({
     const appConfig = projectConfig.apps[0];
 
     const outputPath = serveTaskOptions.outputPath || appConfig.outDir;
+    if (this.project.root === outputPath) {
+      throw new SilentError ('Output path MUST not be project root directory!');
+    }
     rimraf.sync(path.resolve(this.project.root, outputPath));
 
     const serveDefaults = {