Angular-cli: Una pregunta de novato lo siento, ¿cuál es el uso de spec.ts?

Tengo un problema para crear mi ejemplo de proyecto en app.component.ts

import { Component } from '@angular/core';

@Component({
  moduleId: module.id,
  selector: 'app-root',
  templateUrl: 'app.component.html',
  styleUrls: ['app.component.css']
})
export class AppComponent {
  title = 'app works!';
}

si elimino el título = '¡la aplicación funciona!';

ng build fallará y la salida de error es

ng b -dev

Build failed.
The Broccoli Plugin: [BroccoliTypeScriptCompiler] failed with:
Error: Typescript found the following errors:
  /Users/DarkerThanBlack/Projects/html/ng2-admin/tmp/broccoli_type_script_compiler-input_base_path-y9aTpdto.tmp/0/src/app/app.component.spec.ts (21, 16): Property 'title' does not exist on type 'AppComponent'.
    at BroccoliTypeScriptCompiler._doIncrementalBuild (/Users/DarkerThanBlack/Projects/html/ng2-admin/node_modules/angular-cli/lib/broccoli/broccoli-typescript.js:120:19)
    at BroccoliTypeScriptCompiler.build (/Users/DarkerThanBlack/Projects/html/ng2-admin/node_modules/angular-cli/lib/broccoli/broccoli-typescript.js:43:10)
    at /Users/DarkerThanBlack/Projects/html/ng2-admin/node_modules/angular-cli/node_modules/broccoli-caching-writer/index.js:152:21
    at lib$rsvp$$internal$$tryCatch (/Users/DarkerThanBlack/Projects/html/ng2-admin/node_modules/angular-cli/node_modules/broccoli-caching-writer/node_modules/rsvp/dist/rsvp.js:1036:16)
    at lib$rsvp$$internal$$invokeCallback (/Users/DarkerThanBlack/Projects/html/ng2-admin/node_modules/angular-cli/node_modules/broccoli-caching-writer/node_modules/rsvp/dist/rsvp.js:1048:17)
    at lib$rsvp$$internal$$publish (/Users/DarkerThanBlack/Projects/html/ng2-admin/node_modules/angular-cli/node_modules/broccoli-caching-writer/node_modules/rsvp/dist/rsvp.js:1019:11)
    at lib$rsvp$asap$$flush (/Users/DarkerThanBlack/Projects/html/ng2-admin/node_modules/angular-cli/node_modules/broccoli-caching-writer/node_modules/rsvp/dist/rsvp.js:1198:9)
    at _combinedTickCallback (internal/process/next_tick.js:67:7)
    at process._tickCallback (internal/process/next_tick.js:98:9)

The broccoli plugin was instantiated at:
    at BroccoliTypeScriptCompiler.Plugin (/Users/DarkerThanBlack/Projects/html/ng2-admin/node_modules/angular-cli/node_modules/broccoli-caching-writer/node_modules/broccoli-plugin/index.js:10:31)
    at BroccoliTypeScriptCompiler.CachingWriter [as constructor] (/Users/DarkerThanBlack/Projects/html/ng2-admin/node_modules/angular-cli/node_modules/broccoli-caching-writer/index.js:21:10)
    at BroccoliTypeScriptCompiler (/Users/DarkerThanBlack/Projects/html/ng2-admin/node_modules/angular-cli/lib/broccoli/broccoli-typescript.js:27:5)
    at Angular2App._getTsTree (/Users/DarkerThanBlack/Projects/html/ng2-admin/node_modules/angular-cli/lib/broccoli/angular2-app.js:321:18)
    at Angular2App._buildTree (/Users/DarkerThanBlack/Projects/html/ng2-admin/node_modules/angular-cli/lib/broccoli/angular2-app.js:116:23)
    at new Angular2App (/Users/DarkerThanBlack/Projects/html/ng2-admin/node_modules/angular-cli/lib/broccoli/angular2-app.js:53:23)
    at module.exports (/Users/DarkerThanBlack/Projects/html/ng2-admin/angular-cli-build.js:10:10)
    at Class.module.exports.Task.extend.setupBroccoliBuilder (/Users/DarkerThanBlack/Projects/html/ng2-admin/node_modules/angular-cli/node_modules/angular-cli/lib/models/builder.js:55:19)
    at Class.module.exports.Task.extend.init (/Users/DarkerThanBlack/Projects/html/ng2-admin/node_modules/angular-cli/node_modules/angular-cli/lib/models/builder.js:89:10)
    at new Class (/Users/DarkerThanBlack/Projects/html/ng2-admin/node_modules/angular-cli/node_modules/angular-cli/node_modules/core-object/core-object.js:18:12)
    at Class.module.exports.Task.extend.run (/Users/DarkerThanBlack/Projects/html/ng2-admin/node_modules/angular-cli/node_modules/angular-cli/lib/tasks/build.js:15:19)
    at /Users/DarkerThanBlack/Projects/html/ng2-admin/node_modules/angular-cli/node_modules/angular-cli/lib/commands/build.js:32:24
    at lib$rsvp$$internal$$tryCatch (/Users/DarkerThanBlack/Projects/html/ng2-admin/node_modules/angular-cli/node_modules/angular-cli/node_modules/rsvp/dist/rsvp.js:1036:16)
    at lib$rsvp$$internal$$invokeCallback (/Users/DarkerThanBlack/Projects/html/ng2-admin/node_modules/angular-cli/node_modules/angular-cli/node_modules/rsvp/dist/rsvp.js:1048:17)
    at /Users/DarkerThanBlack/Projects/html/ng2-admin/node_modules/angular-cli/node_modules/angular-cli/node_modules/rsvp/dist/rsvp.js:331:11
    at lib$rsvp$asap$$flush (/Users/DarkerThanBlack/Projects/html/ng2-admin/node_modules/angular-cli/node_modules/angular-cli/node_modules/rsvp/dist/rsvp.js:1198:9)