feat: Added warning when modules is not false

When webpack is version 2 or higher and we're using es2015 or env preset
We throw a warning saying that webpack can't treeshake correctly

Closes #477

src/index.js

@@ -9,6 +9,10 @@ const resolveRc = require("./resolve-rc.js");
 const pkg = require("../package.json");
 const fs = require("fs");
 
+const presetsToCheckForModules = ["es2015", "env"];
+const modulesWarningMsg = `We've noticted the option modules isn't set to false,
+which disables treeshaking.`;
+
 /**
  * Error thrown by Babel formatted to conform to Webpack reporting.
  */
@@ -149,6 +153,28 @@ module.exports = function(source, inputSourceMap) {
     options.sourceFileName = relative(options.sourceRoot, options.filename);
   }
 
+  if (this.version > 1 && options.presets) {
+    options.presets.forEach(preset => {
+      let name = preset;
+      let opts = {};
+
+      // if we have extra options put it in opts
+      if (Array.isArray(preset)) {
+        name = preset[0];
+        opts = preset[1];
+      }
+
+      if (
+        presetsToCheckForModules.indexOf(name) > -1 &&
+        (!opts ||
+          (opts && !opts.hasOwnProperty("modules")) ||
+          (opts && opts.modules))
+      ) {
+        this.emitWarning(new Error(modulesWarningMsg));
+      }
+    });
+  }
+
   const cacheDirectory = options.cacheDirectory;
   const cacheIdentifier = options.cacheIdentifier;
   const metadataSubscribers = options.metadataSubscribers;

test/loader.test.js

@@ -263,3 +263,39 @@ test.cb("should not throw without config", t => {
     t.end();
   });
 });
+
+test.cb("should throw a warning when modules is set to false", t => {
+  const config = {
+    entry: path.join(__dirname, "fixtures/basic.js"),
+    output: {
+      path: t.context.directory,
+    },
+    module: {
+      loaders: [
+        {
+          test: /\.jsx?/,
+          loader: babelLoader,
+          exclude: /node_modules/,
+          query: {
+            presets: [["env", { modules: "umd" }]],
+          },
+        },
+      ],
+    },
+  };
+
+  webpack(config, (err, stats) => {
+    t.is(err, null);
+
+    const warnings = stats.compilation.warnings;
+    t.true(warnings.length > 0);
+    if (warnings.length) {
+      t.is(
+        warnings[0].message,
+        "We've noticted the option modules isn't set to false,\nwhich disables treeshaking.",
+      );
+    }
+
+    t.end();
+  });
+});