Warn if running plugin from source

includes/namespace.php

@@ -26,6 +26,8 @@
 
 namespace Google\Web_Stories;
 
+global $heading, $body;
+
 /**
  * Handles plugin activation.
  *
@@ -80,6 +82,60 @@ function deactivate( $network_wide ) {
 register_activation_hook( WEBSTORIES_PLUGIN_FILE, '\Google\Web_Stories\activate' );
 register_deactivation_hook( WEBSTORIES_PLUGIN_FILE, '\Google\Web_Stories\deactivate' );
 
+global $heading;
+global $body;
+
+$heading = __( 'Web Stories plugin could not be initialized.', 'web-stories' );
+$body    = sprintf(
+	/* translators: %s: build commands. */
+	__( 'You appear to be running an incomplete version of the plugin. Please run %s to finish installation.', 'web-stories' ),
+	'<code>composer install &amp;&amp; npm install &amp;&amp; npm run build</code>'
+);
+
+if (
+	! class_exists( '\Google\Web_Stories\Plugin' ) ||
+	! file_exists( WEBSTORIES_PLUGIN_DIR_PATH . '/assets/js/edit-story.js' )
+) {
+	/**
+	 * Displays an admin notice about why the plugin is unable to load.
+	 */
+	function _print_missing_build_admin_notice() {
+		global $heading, $body;
+		?>
+		<div class="notice notice-error">
+			<p>
+				<strong><?php echo esc_html( $heading ); ?></strong>
+			</p>
+			<p>
+				<?php echo wp_kses_post( $body ); ?>
+			</p>
+		</div>
+		<?php
+
+		// Don't pollute global space.
+		unset( $heading, $body );
+	}
+
+	add_action( 'admin_notices', __NAMESPACE__ . '\_print_missing_build_admin_notice' );
+}
+
+// In CLI context, existence of the JS files is not required.
+if (
+	! class_exists( '\Google\Web_Stories\Plugin' ) &&
+	( ( defined( 'WP_CLI' ) && WP_CLI ) || 'true' === getenv( 'CI' ) || 'cli' === PHP_SAPI )
+) {
+	$body = html_entity_decode( str_replace( [ '<code>', '</code>' ], '`', $body ), ENT_QUOTES, 'UTF-8' );
+
+	if ( class_exists( '\WP_CLI' ) ) {
+		WP_CLI::warning( "$heading\n$body" );
+	} else {
+		echo "$heading\n$body\n"; // phpcs:disable WordPress.Security.EscapeOutput.OutputNotEscaped
+	}
+
+	// Don't pollute global space.
+	unset( $heading, $body );
+}
+
 global $web_stories;
 
 $web_stories = new Plugin();