Expand the '.env()' API call with more flexibility.

packages/babel-core/src/config/helpers/config-api.js

@@ -1,19 +1,40 @@
 // @flow
 import type { CacheConfigurator, SimpleCacheConfigurator } from "../caching";
 
+type EnvFunction = {
+  (): string,
+  <T>((string) => T): T,
+  (string): boolean,
+  (Array<string>): boolean,
+};
+
 export type PluginAPI = {
   cache: SimpleCacheConfigurator,
-  env: () => string,
+  env: EnvFunction,
   async: () => boolean,
 };
 
 export default function makeAPI(
   cache: CacheConfigurator<{ envName: string }>,
 ): PluginAPI {
+  const env: any = value =>
+    cache.using(data => {
+      if (typeof value === "undefined") return data.envName;
+      if (typeof value === "function") return value(data.envName);
+      if (!Array.isArray(value)) value = [value];
+
+      return value.some(entry => {
+        if (typeof entry !== "string") {
+          throw new Error("Unexpected non-string value");
+        }
+        return entry === data.envName;
+      });
+    });
+
   return {
     cache: cache.simple(),
     // Expose ".env()" so people can easily get the same env that we expose using the "env" key.
-    env: () => cache.using(data => data.envName),
+    env,
     async: () => false,
   };
 }