Switch to using webpack define plugin to inject a global GUTENBERG_PH…

…ASE variable

package.json

@@ -154,7 +154,7 @@
 		"precheck-local-changes": "npm run docs:build",
 		"check-local-changes": "( git diff -U0 | xargs -0 node bin/process-git-diff ) || ( echo \"There are local uncommitted changes after one or both of 'npm install' or 'npm run docs:build'!\" && exit 1 );",
 		"predev": "npm run check-engines",
-		"dev": "npm run build:packages && concurrently \"wp-scripts start\" \"npm run dev:packages\"",
+		"dev": "cross-env GUTENBERG_PHASE=2 npm run build:packages && concurrently \"wp-scripts start\" \"npm run dev:packages\"",
 		"dev:packages": "node ./bin/packages/watch.js",
 		"docs:build": "node docs/tool",
 		"fixtures:clean": "rimraf \"test/integration/full-content/fixtures/*.+(json|serialized.html)\"",
@@ -168,7 +168,7 @@
 		"lint-pkg-json": "wp-scripts lint-pkg-json ./packages",
 		"lint-css": "wp-scripts lint-style '**/*.scss'",
 		"lint-css:fix": "npm run lint-css -- --fix",
-		"package-plugin": "./bin/build-plugin-zip.sh",
+		"package-plugin": "cross-env GUTENBERG_PHASE=2 ./bin/build-plugin-zip.sh",
 		"postinstall": "npm run check-licenses && npm run build:packages",
 		"pot-to-php": "./bin/pot-to-php.js",
 		"precommit": "lint-staged",

webpack.config.js

@@ -1,6 +1,7 @@
 /**
  * External dependencies
  */
+const { DefinePlugin } = require( 'webpack' );
 const WebpackRTLPlugin = require( 'webpack-rtl-plugin' );
 const LiveReloadPlugin = require( 'webpack-livereload-plugin' );
 const CopyWebpackPlugin = require( 'copy-webpack-plugin' );
@@ -105,6 +106,10 @@ const config = {
 		],
 	},
 	plugins: [
+		// Inject the `GUTENBERG_PHASE` global, used for feature flagging.
+		new DefinePlugin( {
+			GUTENBERG_PHASE: JSON.stringify( process.env.GUTENBERG_PHASE || 1 ),
+		} ),
 		// Create RTL files with a -rtl suffix
 		new WebpackRTLPlugin( {
 			suffix: '-rtl',