Adjust the checkIfOnline check if in a corporate proxy environment (#2884)

* Adjust the `checkIfOnline` check if in a corporate proxy environment
If the environment variable `https_proxy` is set, test that the proxy name is resolveable rather than 'registry.yarnpkg.com'.
This fixes #2832.

* Adjust to check yarnpkg.com first, then check the proxy address only if that failed

Author: bsyk

packages/create-react-app/createReactApp.js

@@ -47,6 +47,7 @@ const semver = require('semver');
 const dns = require('dns');
 const tmp = require('tmp');
 const unpack = require('tar-pack').unpack;
+const url = require('url');
 const hyperquest = require('hyperquest');
 
 const packageJson = require('./package.json');
@@ -614,7 +615,15 @@ function checkIfOnline(useYarn) {
 
   return new Promise(resolve => {
     dns.lookup('registry.yarnpkg.com', err => {
-      resolve(err === null);
+      if (err != null && process.env.https_proxy) {
+        // If a proxy is defined, we likely can't resolve external hostnames.
+        // Try to resolve the proxy name as an indication of a connection.
+        dns.lookup(url.parse(process.env.https_proxy).hostname, proxyErr => {
+          resolve(proxyErr == null);
+        });
+      } else {
+        resolve(err == null);
+      }
     });
   });
 }