Build: Add pragma check webpack plugin for server side rendering

Add a webpack plugin that:
- Scans module sources for the ssr-ready pragma
- Scans non-npm dependencies of these modules, and checks that they also have
  the ssr-ready pragma. Adds a webpack compilation error if not.
- Bails the process if errors are found(!)

server/pragma-checker/README.md

@@ -0,0 +1,7 @@
+PragmaCheckPlugin
+================
+
+This `webpack` plugin scans Calypso source code for the `@ssr-ready` pragma, and reports any dependencies that don't have this pragma.
+
+It is intended to provide immediate runtime feedback to developers modifying components used in Server Side Rendering, where adding a non-compatible dependency may break the server build.
+

server/pragma-checker/index.js

@@ -0,0 +1,72 @@
+/***** WARNING: ES5 code only here. Not transpiled! *****/
+
+/**
+ * External dependecies
+ */
+var error = require( 'chalk' ).bold.red;
+var info = require( 'chalk' ).bold.yellow;
+var startsWith = require( 'lodash/string/startsWith' );
+var includes = require( 'lodash/collection/includes' );
+
+var PLUGIN_TITLE = 'PragmaChecker';
+var SSR_READY = '/** @ssr-ready **/';
+
+function PragmaCheckPlugin( options ) {
+	this.options = options || {};
+}
+
+function scanDependencies( module, compilation ) {
+	if ( ! module.dependencies ) {
+		return;
+	}
+
+	module.dependencies.forEach( function( dep ) {
+		if ( ! dep.module ) {
+			return;
+		}
+
+		// If the module is compiled through babel, we can be pretty sure it's our own module, not from npm.
+		if ( includes( dep.module.request, 'babel-loader' ) &&
+				dep.module._source &&
+				! includes( dep.module._source._value, SSR_READY ) ) {
+			compilation.errors.push( PLUGIN_TITLE + ': ' + module.rawRequest + ', dependency ' + dep.module.rawRequest + ' is not ' + SSR_READY );
+		}
+
+		if ( dep.module.dependencies ) {
+			scanDependencies( dep.module.dependencies );
+		}
+	} );
+}
+
+PragmaCheckPlugin.prototype.apply = function( compiler ) {
+	compiler.plugin( 'compilation', function( compilation ) {
+		compilation.plugin( 'optimize-modules', function( modules ) {
+			modules.forEach( function( module ) {
+				if ( module._source && includes( module._source._value, SSR_READY ) ) {
+					scanDependencies( module, compilation );
+				}
+			} );
+		} );
+
+		compiler.plugin( 'done', function( stats ) {
+			var pragmaError = false;
+
+			if ( stats.compilation.errors && stats.compilation.errors.length ) {
+				stats.compilation.errors.forEach( function( text ) {
+					if ( startsWith( text, PLUGIN_TITLE ) ) {
+						pragmaError = true;
+						console.log( error( text ) );
+					}
+				} );
+
+				if ( pragmaError ) {
+					console.log( info( PLUGIN_TITLE + ': Server Side Rendering constraints not met. Please see https://github.com/Automattic/wp-calypso/tree/master/shared#shared for details.' ) );
+					// We don't want to enable webpack's bail for all errors, so exit the process instead of throwing.
+					process.exit( 1 ); // eslint-disable-line no-process-exit
+				}
+			}
+		} );
+	} );
+};
+
+module.exports = PragmaCheckPlugin;

webpack.config.js

@@ -10,7 +10,8 @@ var webpack = require( 'webpack' ),
  * Internal dependencies
  */
 var config = require( './server/config' ),
-	ChunkFileNamePlugin = require( './server/bundler/plugin' );
+	ChunkFileNamePlugin = require( './server/bundler/plugin' ),
+	PragmaCheckPlugin = require( 'server/pragma-checker' );
 
 /**
  * Internal variables
@@ -96,6 +97,7 @@ jsLoader = {
 };
 
 if ( CALYPSO_ENV === 'development' ) {
+	webpackConfig.plugins.push( new PragmaCheckPlugin() );
 	webpackConfig.plugins.push( new webpack.HotModuleReplacementPlugin() );
 	webpackConfig.entry[ 'build-' + CALYPSO_ENV ] = [
 		'webpack-dev-server/client?/',