fix(@angular/cli): Throw error when no key provided for ng get

Close #5887

Author: prestonvanloon

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

@@ -11,7 +11,7 @@ export interface GetOptions {
 
 const GetCommand = Command.extend({
   name: 'get',
-  description: 'Get a value from the configuration.',
+  description: 'Get a value from the configuration. Example: ng get [key]',
   works: 'everywhere',
 
   availableOptions: [
@@ -32,6 +32,10 @@ const GetCommand = Command.extend({
           + 'you need the --global argument.');
       }
 
+      if (!rawArgs[0]) {
+        throw new SilentError('No key specified. Run "ng help get" for usage.');
+      }
+
       const value = config.get(rawArgs[0]);
 
       if (value === null || value === undefined) {

tests/e2e/tests/commands/get/get.ts

@@ -4,6 +4,7 @@ import {expectToFail} from '../../../utils/utils';
 export default function() {
   return Promise.resolve()
     .then(() => expectToFail(() => ng('get', 'apps.zzz.prefix')))
+    .then(() => expectToFail(() => ng('get')))
     .then(() => ng('get', 'apps.0.prefix'))
     .then(({ stdout }) => {
       if (!stdout.match(/app/)) {