Copy rather than mutating .babelrc configuration objects.

Helps with #9469, since any extraneous "env" property will be removed from
the copied object.

packages/babel-compiler/babel-compiler.js

@@ -222,25 +222,40 @@ BCp._inferHelper = function (
 
  function walkBabelRC(obj, path) {
  if (obj && typeof obj === "object") {
+ const copy = Object.create(null);
+
  path = path || [];
- var index = path.push("presets") - 1;
- walkHelper(obj.presets, path);
- path[index] = "plugins";
- walkHelper(obj.plugins, path);
+ const index = path.length;
+
+ if (obj.presets) {
+ path[index] = "presets";
+ copy.presets = walkHelper(obj.presets, path);
+ }
+
+ if (obj.plugins) {
+ path[index] = "plugins";
+ copy.plugins = walkHelper(obj.plugins, path);
+ }
+
  path.pop();
+
+ return copy;
  }
+
+ return obj;
  }
 
  function walkHelper(list, path) {
- if (list) {
- // Empty the list and then refill it with resolved values.
- list.splice(0).forEach(function (pluginOrPreset) {
- var res = resolveHelper(pluginOrPreset, path);
- if (res) {
- list.push(res);
- }
- });
- }
+ const copy = [];
+
+ list.forEach(function (pluginOrPreset) {
+ const res = resolveHelper(pluginOrPreset, path);
+ if (res) {
+ copy.push(res);
+ }
+ });
+
+ return copy;
  }
 
  function resolveHelper(value, path) {
@@ -252,24 +267,24 @@ BCp._inferHelper = function (
 
  if (Array.isArray(value)) {
  // The value is a [plugin, options] pair.
- var res = value[0] = resolveHelper(value[0], path);
+ const res = resolveHelper(value[0], path);
  if (res) {
- return value;
+ const copy = value.slice(0);
+ copy[0] = res;
+ return copy;
  }
 
  } else if (typeof value === "string") {
  // The value is a string that we need to require.
- var result = requireWithPath(value, path);
+ const result = requireWithPath(value, path);
  if (result && result.module) {
  cacheDeps[result.name] = result.version;
- walkBabelRC(result.module, path);
- return result.module;
+ return walkBabelRC(result.module, path);
  }
 
  } else if (typeof value === "object") {
  // The value is a { presets?, plugins? } preset object.
- walkBabelRC(value, path);
- return value;
+ return walkBabelRC(value, path);
  }
  }
 
@@ -310,25 +325,26 @@ BCp._inferHelper = function (
  }
  }
 
- babelrc = JSON.parse(JSON.stringify(babelrc));
+ const clean = walkBabelRC(babelrc);
+ merge(babelOptions, clean, "presets");
+ merge(babelOptions, clean, "plugins");
 
- walkBabelRC(babelrc);
+ if (babelrc && babelrc.env) {
+ const envKey =
+ process.env.BABEL_ENV ||
+ process.env.NODE_ENV ||
+ "development";
 
- merge(babelOptions, babelrc, "presets");
- merge(babelOptions, babelrc, "plugins");
+ const clean = walkBabelRC(babelrc.env[envKey]);
 
- const babelEnv = (process.env.BABEL_ENV ||
- process.env.NODE_ENV ||
- "development");
- if (babelrc && babelrc.env && babelrc.env[babelEnv]) {
- const env = babelrc.env[babelEnv];
- walkBabelRC(env);
- merge(babelOptions, env, "presets");
- merge(babelOptions, env, "plugins");
+ if (clean) {
+ merge(babelOptions, clean, "presets");
+ merge(babelOptions, clean, "plugins");
+ }
  }
 
- return !! (babelrc.presets ||
- babelrc.plugins);
+ return !! (babelOptions.presets ||
+ babelOptions.plugins);
 };
 
 function merge(babelOptions, babelrc, name) {