chore: Source babel-preset-env targets from package.json

- Add .babelrc.js and use the browserslist defined in package.json to set
  targets for babel-preset-env.
- Reference .babelrc.js from .babelrc file because babel@6 does not support
  common js module for defining config.

template/.babelrc

@@ -1,19 +1,8 @@
 {
-  "presets": [
-    ["env", {
-      "modules": false,
-      "targets": {
-        "browsers": ["> 1%", "last 2 versions", "not ie <= 8"]
-      }
-    }],
-    "stage-2"
-  ],
-  "plugins": ["transform-runtime"]{{#if_or unit e2e}},
+  "presets": ["./.babelrc.js"]{{#if_or unit e2e}},
   "env": {
     "test": {
-      "presets": ["env", "stage-2"]{{#if_eq runner "karma"}},
-      "plugins": ["istanbul"]{{/if_eq}}{{#if_eq runner "jest"}},
-      "plugins": ["transform-es2015-modules-commonjs", "dynamic-import-node"]{{/if_eq}}
+      "presets": ["./.babelrc.js"]
     }
   }{{/if_or}}
 }

template/.babelrc.js

@@ -0,0 +1,29 @@
+'use strict';
+
+const pkg = require('./package.json')
+
+module.exports = function babelConfig() { {{#if_or unit e2e}}
+  if (process.env.NODE_ENV === 'test') {
+    return {
+      presets: ['env', 'stage-2']{{#if_eq runner "karma"}},
+      plugins: ['istanbul']{{/if_eq}}{{#if_eq runner "jest"}},
+      plugins: ['transform-es2015-modules-commonjs', 'dynamic-import-node']{{/if_eq}}
+    }
+  }{{/if_or}}
+  return {
+    presets: [
+      [
+        'env',
+        {
+          modules: false,
+          // This shouldn't be necessary once `@babel/preset-env` is out of beta.
+          targets: {
+            browsers: pkg.browserslist
+          }
+        }
+      ],
+      'stage-2'
+    ],
+    plugins: ['transform-runtime']
+  }
+}