fix(@angular/cli): delete only dist content

Fix #5925

Author: filipesilva

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

@@ -27,7 +27,7 @@ export default Task.extend({
     if (config.project && config.project.ejected) {
       throw new SilentError('An ejected project cannot use the build command anymore.');
     }
-    rimraf.sync(path.resolve(project.root, outputPath));
+    rimraf.sync(`${path.resolve(project.root, outputPath)}/*`);
 
     const webpackConfig = new NgCliWebpackConfig(runTaskOptions, app).buildConfig();
     const webpackCompiler = webpack(webpackConfig);

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

@@ -31,7 +31,7 @@ export default Task.extend({
     if (projectConfig.project && projectConfig.project.ejected) {
       throw new SilentError('An ejected project cannot use the build command anymore.');
     }
-    rimraf.sync(path.resolve(this.project.root, outputPath));
+    rimraf.sync(`${path.resolve(this.project.root, outputPath)}/*`);
 
     const serveDefaults = {
       // default deployUrl to '' on serve to prevent the default from .angular-cli.json

tests/e2e/tests/build/fail-build.ts

@@ -6,6 +6,6 @@ export default function() {
   return deleteFile('src/app/app.component.ts')
     // This is supposed to fail since there's a missing file
     .then(() => expectToFail(() => ng('build')))
-    // Failed builds don't leave behind dist/
-    .then(() => expectToFail(() => expectFileToExist('dist/')));
+    // Failed builds don't leave behind files inside dist/
+    .then(() => expectToFail(() => expectFileToExist('dist/main.bundle.js')));
 }