Upgrade gulp v4 (#6143)

* deps: update dependency gulp to ^4.0.0

*  deps: drop deprecated gulp-util dependency

* deps: drop gulp-watch dependency

Gulp v4 integrates a watch mecanism, there is no need
for gulp-watch anymore.

gulpfile.js

@@ -5,8 +5,7 @@ const plumber = require('gulp-plumber');
 const newer = require('gulp-newer');
 const babel = require('gulp-babel');
 const sourcemaps = require('gulp-sourcemaps');
-const watch = require('gulp-watch');
-const gutil = require('gulp-util');
+const log = require('fancy-log');
 const gulpif = require('gulp-if');
 const gulp = require('gulp');
 const path = require('path');
@@ -21,7 +20,7 @@ const build = (lib, opts) =>
   gulp.src('src/**/*.js')
       .pipe(plumber({
         errorHandler(err) {
-          gutil.log(err.stack);
+          log.error(err.stack);
         },
       }))
       .pipe(newer(lib))
@@ -30,14 +29,13 @@ const build = (lib, opts) =>
       .pipe(gulpif(argv.sourcemaps, sourcemaps.write('.', {sourceRoot: '../src'})))
       .pipe(gulp.dest(lib));
 
-gulp.task('default', ['build']);
-
 gulp.task('build', () =>
   build('lib', babelRc.env[majorVer >= 5 ? 'node5' : 'pre-node5'])
 );
 
-gulp.task('watch', ['build'], () => {
-  watch('src/**/*', () => {
-    gulp.start('build');
-  });
-});
+gulp.task('default', gulp.task('build'));
+
+gulp.task(
+  'watch',
+  gulp.series('build', () => gulp.watch('src/**/*', gulp.task('build')))
+);

package.json

@@ -79,16 +79,15 @@
     "eslint-plugin-relay": "^0.0.24",
     "eslint-plugin-yarn-internal": "file:scripts/eslint-rules",
     "execa": "^0.10.0",
+    "fancy-log": "^1.3.2",
     "flow-bin": "^0.66.0",
     "git-release-notes": "^3.0.0",
-    "gulp": "^3.9.0",
+    "gulp": "^4.0.0",
     "gulp-babel": "^7.0.0",
     "gulp-if": "^2.0.1",
     "gulp-newer": "^1.0.0",
     "gulp-plumber": "^1.0.1",
     "gulp-sourcemaps": "^2.2.0",
-    "gulp-util": "^3.0.7",
-    "gulp-watch": "^5.0.0",
     "jest": "^22.4.4",
     "jsinspect": "^0.12.6",
     "minimatch": "^3.0.4",