Disallow nested '.env' blocks since they are useless.

packages/babel-core/src/config/options.js

@@ -206,6 +206,9 @@ export function validate(type: OptionsType, opts: {}): ValidatedOptions {
     if (type !== "arguments" && ROOT_VALIDATORS[key]) {
       throw new Error(`.${key} is only allowed in root programmatic options`);
     }
+    if (type === "env" && key === "env") {
+      throw new Error(`.${key} is not allowed inside another env block`);
+    }
 
     const validator =
       COMMON_VALIDATORS[key] ||

packages/babel-core/test/api.js

@@ -296,12 +296,6 @@ describe("api", function() {
         development: {
           passPerPreset: true,
           presets: [pushPreset("argthree"), pushPreset("argfour")],
-          env: {
-            development: {
-              passPerPreset: true,
-              presets: [pushPreset("argfive"), pushPreset("argsix")],
-            },
-          },
         },
       },
     });
@@ -327,8 +321,6 @@ describe("api", function() {
         "fourteen;",
         "fifteen;",
         "sixteen;",
-        "argfive;",
-        "argsix;",
         "argthree;",
         "argfour;",
         "seven;",