refactor(@angular/cli): refactor isTTY() function

It was really hard to read with double negatives and such.

Author: hansl

packages/angular/cli/utilities/tty.ts

@@ -5,13 +5,18 @@
  * Use of this source code is governed by an MIT-style license that can be
  * found in the LICENSE file at https://angular.io/license
  */
+
+function _isTruthy(value: undefined | string): boolean {
+  // Returns true if value is a string that is anything but 0 or false.
+  return value !== undefined && value !== '0' && value.toUpperCase() !== 'FALSE';
+}
+
 export function isTTY(): boolean {
+  // If we force TTY, we always return true.
   const force = process.env['NG_FORCE_TTY'];
   if (force !== undefined) {
-    return !(force === '0' || force.toUpperCase() === 'FALSE');
+    return _isTruthy(force);
   }
 
-  const ci = process.env['CI'];
-
-  return !!process.stdout.isTTY && (!ci || ci === '0' || ci.toUpperCase() === 'FALSE');
+  return !!process.stdout.isTTY && !_isTruthy(process.env['CI']);
 }