Update babelrc with comments

.babelrc

-{
-  "presets": [
-    [
-      "@babel/preset-env",
-      {
-        "modules": false,
-        "targets": {
-          "ie": "11"
-        }
-      }
-    ]
-  ],
-  "plugins": [
-    "@babel/plugin-syntax-dynamic-import",
-    "@babel/plugin-syntax-import-meta",
-    "@babel/plugin-proposal-class-properties",
-    "@babel/plugin-proposal-json-strings"
-  ],
-  "env": {
-    "karma": {
-      "plugins": ["babel-plugin-rewire"]
-    },
-    "coverage": {
-      "plugins": [
-        [
-          "babel-plugin-istanbul",
-          {
-            "exclude": ["spec/javascripts/**/*", "app/assets/javascripts/locale/**/app.js"]
-          }
-        ],
-        "babel-plugin-rewire"
-      ]
-    }
-  }
-}

.babelrc.js

+const BABEL_ENV = process.env.BABEL_ENV || process.env.NODE_ENV || null;
+
+const presets = [
+  [
+    '@babel/preset-env',
+    {
+      modules: false,
+      targets: {
+        ie: '11',
+      },
+    },
+  ],
+];
+
+// include stage 3 proposals
+const plugins = [
+  '@babel/plugin-syntax-dynamic-import',
+  '@babel/plugin-syntax-import-meta',
+  '@babel/plugin-proposal-class-properties',
+  '@babel/plugin-proposal-json-strings',
+];
+
+// add code coverage tooling if necessary
+if (BABEL_ENV === 'coverage') {
+  plugins.push([
+    'babel-plugin-istanbul',
+    {
+      exclude: ['spec/javascripts/**/*', 'app/assets/javascripts/locale/**/app.js'],
+    },
+  ]);
+}
+
+// add rewire support when running tests
+if (BABEL_ENV === 'karma' || BABEL_ENV === 'coverage') {
+  plugins.push('babel-plugin-rewire');
+}
+
+module.exports = { presets, plugins };