Properly handle class properties proposal

Historically, we only add the `@babel/plugin-proposal-class-properties`
so that we make sure the ordering is right with the decorators proposal
(otherwise, it can end up compiling in the wrong order). With a recent
version of `@babel/preset-env` and, transitively, `caniuse-lite`, this
resulted in cases where we added that plugin but *not* related plugins
for private class properties, which in turn triggered a Babel assertion
about not adding the properties together as appropriate when the caniuse
database (correctly) reported that .

The fix is:

1.  Bump to a more recent version of `@babel/preset-env`, which comes
    with a correspondingly bumped version of `caniuse-lite`, which in
    turn correctly understands what the latest versions of targeted
    browsers are.

2.  Include in `ember-cli-babel` itself a check for whether we even
    *need* to add the plugin, and only provide it when the provided
    `targets` indicate that they require it.

Resolves #419

lib/babel-options-util.js

@@ -30,8 +30,8 @@ function _getPresetEnv(config, project) {
 }
 
 function _getModulesPlugin() {
-  const resolvePath = require("./relative-module-paths")
-    .resolveRelativeModulePath;
+  const resolvePath =
+    require("./relative-module-paths").resolveRelativeModulePath;
 
   return [
     [require.resolve("babel-plugin-module-resolver"), { resolvePath }],
@@ -296,7 +296,12 @@ function _getHelperVersion(project) {
   return APP_BABEL_RUNTIME_VERSION.get(project);
 }
 
-function _buildClassFeaturePluginConstraints(constraints, config, parent, project) {
+function _buildClassFeaturePluginConstraints(
+  constraints,
+  config,
+  parent,
+  project
+) {
   // With versions of ember-cli-typescript < 4.0, class feature plugins like
   // @babel/plugin-proposal-class-properties were run before the TS transform.
   if (!_shouldHandleTypeScript(config, parent, project)) {
@@ -307,7 +312,14 @@ function _buildClassFeaturePluginConstraints(constraints, config, parent, projec
   return constraints;
 }
 
-function _addDecoratorPlugins(plugins, options, config, parent, project) {
+function _addDecoratorPlugins({
+  plugins,
+  options,
+  config,
+  parent,
+  project,
+  isClassPropertiesRequired,
+}) {
   const { hasPlugin, addPlugin } = require("ember-cli-babel-plugin-helpers");
 
   if (hasPlugin(plugins, "@babel/plugin-proposal-decorators")) {
@@ -341,7 +353,7 @@ function _addDecoratorPlugins(plugins, options, config, parent, project) {
         )} has added the class-properties plugin to its build, but ember-cli-babel provides these by default now! You can remove the transforms, or the addon that provided them, such as @ember-decorators/babel-transforms.`
       );
     }
-  } else {
+  } else if (isClassPropertiesRequired) {
     addPlugin(
       plugins,
       [

lib/get-babel-options.js

@@ -14,14 +14,14 @@ const {
   _getPresetEnv,
 } = require("./babel-options-util");
 
-module.exports = function getBabelOptions(config, appInstance) {
-  let { parent, project } = appInstance;
+module.exports = function getBabelOptions(config, cliBabelInstance) {
+  let { parent, project } = cliBabelInstance;
   let addonProvidedConfig = _getAddonProvidedConfig(config);
-  let shouldIncludeHelpers = _shouldIncludeHelpers(config, appInstance);
+  let shouldIncludeHelpers = _shouldIncludeHelpers(config, cliBabelInstance);
   let shouldHandleTypeScript = _shouldHandleTypeScript(config, parent, project);
   let shouldIncludeDecoratorPlugins = _shouldIncludeDecoratorPlugins(config);
 
- let emberCLIBabelConfig = config["ember-cli-babel"];
+  let emberCLIBabelConfig = config["ember-cli-babel"];
   let shouldRunPresetEnv = true;
 
   if (emberCLIBabelConfig) {
@@ -38,13 +38,16 @@ module.exports = function getBabelOptions(config, appInstance) {
   }
 
   if (shouldIncludeDecoratorPlugins) {
-    userPlugins = _addDecoratorPlugins(
-      userPlugins.slice(),
-      addonProvidedConfig.options,
+    userPlugins = _addDecoratorPlugins({
+      plugins: userPlugins.slice(),
+      options: addonProvidedConfig.options,
       config,
       parent,
-      project
-    );
+      project,
+      isClassPropertiesRequired: cliBabelInstance.isPluginRequired(
+        "proposal-class-properties"
+      ),
+    });
   }
 
   options.plugins = []
@@ -56,7 +59,8 @@ module.exports = function getBabelOptions(config, appInstance) {
       _getEmberDataPackagesPolyfill(config, parent),
       _shouldCompileModules(config, project) && _getModulesPlugin(),
       userPostTransformPlugins
-    ).filter(Boolean);
+    )
+    .filter(Boolean);
 
   options.presets = [
     shouldRunPresetEnv && _getPresetEnv(addonProvidedConfig, project),