fix(serve): delete dist on serve (#4293)

`ng serve` will serve the app from memory, but it's easy to think that it just serves what's in `dist/`.

This way, `dist/` is deleted on `ng serve` and users will not have a `dist/` dir where they might think the files are being served from.

Partially address #4290

packages/angular-cli/tasks/serve.ts

@@ -1,6 +1,7 @@
 import * as fs from 'fs';
 import * as path from 'path';
 import * as chalk from 'chalk';
+import * as rimraf from 'rimraf';
 const SilentError = require('silent-error');
 const Task = require('../ember-cli/lib/models/task');
 import * as webpack from 'webpack';
@@ -22,6 +23,9 @@ export default Task.extend({
     const projectConfig = CliConfig.fromProject().config;
     const appConfig = projectConfig.apps[0];
 
+    const outputPath = serveTaskOptions.outputPath || appConfig.outDir;
+    rimraf.sync(path.resolve(this.project.root, outputPath));
+
     const serveDefaults = {
       // default deployUrl to '' on serve to prevent the default from angular-cli.json
       deployUrl: ''