upgrade gulp to v4

[nicholascioli]

As of NodeJS v12.4.0, gulp v3 fails to run at all. This small change
should allow for future compatibility with NodeJS.

The basics

• I branched from develop
• My pull request is against develop
• My code follows the style guide

The details

Resolves

Resolves: #2582

Proposed Changes

This pull request will update gulp to v4 and also update the gulpfile to use
the new v4 syntax.

Reason for Changes

Without this change, any attempt to use blockly in a NodeJS (or NodeJS-built) v12.4.0 project fails.

Test Coverage

Tested on:

• Desktop Chrome
• Desktop Firefox
• Desktop Safari

Additional Information

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here (e.g. I signed it!) and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[nicholascioli]

I signed it! Il ven 21 giu 2019, 17:59 googlebot <notifications@github.com> ha scritto:

…

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA). 📝 *Please visit https://cla.developers.google.com/ <https://cla.developers.google.com/> to sign.* Once you've signed (or fixed any issues), please reply here (e.g. I signed it!) and we'll verify it. ------------------------------ What to do if you already signed the CLA Individual signers - It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data <https://cla.developers.google.com/clas> and verify that your email is set on your git commits <https://help.github.com/articles/setting-your-email-in-git/>. Corporate signers - Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version <https://opensource.google.com/docs/cla/#troubleshoot>). - The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data <https://cla.developers.google.com/clas> and verify that your email is set on your git commits <https://help.github.com/articles/setting-your-email-in-git/>. - The email used to register you as an authorized contributor must also be attached to your GitHub account <https://github.com/settings/emails>. ℹ️ *Googlers: Go here <https://goto.google.com/prinfo/https%3A%2F%2Fgithub.com%2Fgoogle%2Fblockly%2Fpull%2F2583> for more info*. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#2583?email_source=notifications&email_token=AAJO3K5HG2WM2FI4BJVNPX3P3VMOXA5CNFSM4H2VKI2KYY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGODYJY7IA#issuecomment-504598432>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAJO3K4DIQCF3HXDXMDS5DDP3VMOXANCNFSM4H2VKI2A> .

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[rachel-fenichel]

Thanks for providing both the bug report and the solution!