Upgrade to gulp@4 (#7811)

Gulpfile.js

@@ -5,7 +5,7 @@ const through = require("through2");
 const chalk = require("chalk");
 const newer = require("gulp-newer");
 const babel = require("gulp-babel");
-const watch = require("gulp-watch");
+const gulpWatch = require("gulp-watch");
 const gutil = require("gulp-util");
 const filter = require("gulp-filter");
 const gulp = require("gulp");
@@ -114,21 +114,26 @@ function buildRollup(packages) {
   );
 }
 
-gulp.task("default", ["build"]);
-
 gulp.task("build", function() {
   const bundles = ["packages/babylon"];
 
   return merge([buildBabel(/* exclude */ bundles), buildRollup(bundles)]);
 });
 
+gulp.task("default", gulp.series("build"));
+
 gulp.task("build-no-bundle", () => buildBabel());
 
-gulp.task("watch", ["build-no-bundle"], function() {
-  watch(sources.map(getGlobFromSource), { debounceDelay: 200 }, function() {
-    gulp.start("build-no-bundle");
-  });
-});
+gulp.task(
+  "watch",
+  gulp.series("build-no-bundle", function watch() {
+    gulpWatch(
+      sources.map(getGlobFromSource),
+      { debounceDelay: 200 },
+      gulp.task("build-no-bundle")
+    );
+  })
+);
 
 registerStandalonePackageTask(
   gulp,

package.json

@@ -35,7 +35,7 @@
     "eslint-plugin-prettier": "^2.5.0",
     "flow-bin": "^0.66.0",
     "graceful-fs": "^4.1.11",
-    "gulp": "^3.9.0",
+    "gulp": "^4.0.0",
     "gulp-babel": "^8.0.0-beta.2",
     "gulp-filter": "^5.1.0",
     "gulp-newer": "^1.0.0",
@@ -43,7 +43,7 @@
     "gulp-rename": "^1.2.2",
     "gulp-uglify": "^3.0.0",
     "gulp-util": "^3.0.7",
-    "gulp-watch": "^4.3.5",
+    "gulp-watch": "^5.0.0",
     "husky": "^0.14.3",
     "jest": "^22.4.2",
     "lerna": "2.0.0",