don't mutate InputTarget's passed to @babel/helper-compilation-targets

If `@babel/helper-compilation-targets` is called directly, as it is in the
[Ember-CLI
babel](https://github.com/babel/ember-cli-babel/blob/6f76f405b9dd2a48cce394c4826dd50847f74282/index.js#L543-L545)
and [Vue
CLI](https://github.com/vuejs/vue-cli/blob/c8cecffedbf7b19cf930bb2821b5c352bc716a67/packages/%40vue/babel-preset-app/index.js#L17)
integrations, we don't want to mutate the input passed in. At least in Ember
CLI, babel could be called multiple times. So, on subsequent passes all
`browsers` and `esmodules` info would be lost (and losing the `browsers` info
affects the output in negative ways.. Copying the object before dropping keys
allows callers not have to worry about babel mutating the object they pass in.
Switching to a more functional approach makes it easier for babel to not worry
about what consumers are passing in.

Dropping the `browser` property was introduced in: #11500
Dropping the `esmodules` property was introduced in #11124

Discussion about dropping the `esmodules` property: #11124 (comment)

packages/babel-helper-compilation-targets/src/index.js

@@ -166,24 +166,26 @@ export default function getTargets(
   options: Object = {},
 ): Targets {
   const targetOpts: Targets = {};
+  let { browsers } = inputTargets;
 
   // `esmodules` as a target indicates the specific set of browsers supporting ES Modules.
   // These values OVERRIDE the `browsers` field.
   if (inputTargets.esmodules) {
     const supportsESModules = browserModulesData["es6.module"];
-    inputTargets.browsers = Object.keys(supportsESModules)
+    browsers = Object.keys(supportsESModules)
       .map(browser => `${browser} ${supportsESModules[browser]}`)
       .join(", ");
   }
 
-  // Remove esmodules after being consumed to fix `hasTargets` below
-  delete inputTargets.esmodules;
-
   // Parse browsers target via browserslist
-  const browsersquery = validateBrowsers(inputTargets.browsers);
-  delete inputTargets.browsers;
+  const browsersquery = validateBrowsers(browsers);
+
+  // Remove esmodules after being consumed to fix `hasTargets` below
+  const input = { ...inputTargets };
+  delete input.esmodules;
+  delete input.browsers;
 
-  let targets: Targets = validateTargetNames(inputTargets);
+  let targets: Targets = validateTargetNames(input);
 
   const shouldParseBrowsers = !!browsersquery;
   const hasTargets = shouldParseBrowsers || Object.keys(targets).length > 0;

packages/babel-helper-compilation-targets/test/targets-parser.spec.js

@@ -20,6 +20,18 @@ describe("getTargets", () => {
     });
   });
 
+  it("does not mutate the input", () => {
+    const input = Object.freeze({ browsers: "defaults", esmodules: true });
+    const expected = getTargets({
+      browsers: browserslist.defaults,
+      esmodules: true,
+    });
+    const actual = getTargets(input);
+    expect(actual).toEqual(expected);
+    expect(input.browsers).toEqual("defaults");
+    expect(input.esmodules).toEqual(true);
+  });
+
   it("allows 'defaults' query", () => {
     const browserslistDefaults = browserslist.defaults;