feat(@angular/cli): add flag to not delete output path

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

@@ -118,6 +118,13 @@ export const baseBuildCommandOptions: any = [
     type: String,
     aliases: ['a'],
     description: 'Specifies app name or index to use.'
+  },
+  {
+    name: 'delete-output-path',
+    type: Boolean,
+    default: true,
+    aliases: ['dop'],
+    description: 'Delete output path before build.'
   }
 ];
 

packages/@angular/cli/models/build-options.ts

@@ -17,5 +17,5 @@ export interface BuildOptions {
   outputHashing?: string;
   poll?: number;
   app?: string;
-
+  deleteOutputPath?: boolean;
 }

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

@@ -27,7 +27,9 @@ 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));
+    if (runTaskOptions.deleteOutputPath) {
+      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 +31,9 @@ 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));
+    if (serveTaskOptions.deleteOutputPath) {
+      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/delete-output-path.ts

@@ -0,0 +1,21 @@
+import {ng} from '../../utils/process';
+import {expectToFail} from '../../utils/utils';
+import {deleteFile, expectFileToExist} from '../../utils/fs';
+import {getGlobalVariable} from '../../utils/env';
+
+export default function() {
+  // Skip this in ejected tests.
+  if (getGlobalVariable('argv').eject) {
+    return Promise.resolve();
+  }
+
+  return ng('build')
+    // This is supposed to fail since there's a missing file
+    .then(() => deleteFile('src/app/app.component.ts'))
+    // The build fails but we don't delete the output of the previous build.
+    .then(() => expectToFail(() => ng('build', '--no-delete-output-path')))
+    .then(() => expectFileToExist('dist'))
+    // By default, output path is always cleared.
+    .then(() => expectToFail(() => ng('build')))
+    .then(() => expectToFail(() => expectFileToExist('dist')));
+}