Remove Babel transformations for older browser versions (#6812)

The Babel config had previously supported all browsers with greater than
0.25% global usage (according to `browserlist`). This resulted in
`babel-preset-env` including plugins sufficient to support the following
minimum browser versions:

```
{
  "chrome": "49",
  "android": "4.4",
  "edge": "16",
  "firefox": "52",
  "ios": "9.3",
  "safari": "11"
}
```

Instead, the babel config now explicitly supports chrome >= 58 and
firefox >= 53. Chrome and Firefox are the only browsers we currently
publish to, and these were the minimum versions with no additional Babel
transformations.

The minimum browser versions we support should be re-evaluated later,
when we have added tests and documentation.

The plugin 'transform-async-to-generator' has also been removed. It was
used to translate async/await, but our browser targets all support
async/await.

Removing some of these transformations exposed bugs in `uglify-es` that
only presented themselves in the production build. `gulp-uglify-es` has
been updated to a version that uses `terser` instead of `uglify-es`,
which has resolved these issues.

Relates to #6805

.babelrc

@@ -5,9 +5,8 @@
       {
         "targets": {
           "browsers": [
-            ">0.25%",
-            "not ie 11",
-            "not op_mini all"
+            "chrome >= 58",
+            "firefox >= 53"
           ]
         }
       }
@@ -16,7 +15,6 @@
   ],
   "plugins": [
     "transform-runtime",
-    "transform-async-to-generator",
     "transform-class-properties",
     "transform-object-rest-spread"
   ]

package-lock.json

@@ -35501,14 +35501,14 @@
       }
     },
     "gulp-uglify-es": {
-      "version": "1.0.1",
-      "resolved": "https://registry.npmjs.org/gulp-uglify-es/-/gulp-uglify-es-1.0.1.tgz",
-      "integrity": "sha512-lwVTOhSPNJvoPZ8wC1mrDiqSszS8INK2R4kaslBKf9wyMVXLWckKs+LRh5bgcymeB5zsczGKT9bGCTNKeEoCvQ==",
+      "version": "1.0.4",
+      "resolved": "https://registry.npmjs.org/gulp-uglify-es/-/gulp-uglify-es-1.0.4.tgz",
+      "integrity": "sha512-UMRufZsBmQizCYpftutaiVoLswpbzFEfY90EJLU4YlTgculeHnanb794s88TMd5tpCZVC638sAX6JrLVYTP/Wg==",
       "dev": true,
       "requires": {
         "o-stream": "^0.2.2",
         "plugin-error": "^1.0.1",
-        "uglify-es": "^3.3.9",
+        "terser": "^3.7.5",
         "vinyl": "^2.1.0",
         "vinyl-sourcemaps-apply": "^0.2.1"
       },
@@ -35520,9 +35520,9 @@
           "dev": true
         },
         "commander": {
-          "version": "2.13.0",
-          "resolved": "https://registry.npmjs.org/commander/-/commander-2.13.0.tgz",
-          "integrity": "sha512-MVuS359B+YzaWqjCL/c+22gfryv+mCBPHAv3zyVI2GN8EY6IRP8VwtasXn8jyyhvvq84R4ImN1OKRtcbIasjYA==",
+          "version": "2.20.0",
+          "resolved": "https://registry.npmjs.org/commander/-/commander-2.20.0.tgz",
+          "integrity": "sha512-7j2y+40w61zy6YC2iRNpUe/NwhNyoXrYpHMrSunaMG64nRnaf96zO/KMQR4OyN/UnE5KLyEBnKHd4aG3rskjpQ==",
           "dev": true
         },
         "extend-shallow": {
@@ -35562,14 +35562,25 @@
           "integrity": "sha512-UjgapumWlbMhkBgzT7Ykc5YXUT46F0iKu8SGXq0bcwP5dz/h0Plj6enJqjz1Zbq2l5WaqYnrVbwWOWMyF3F47g==",
           "dev": true
         },
-        "uglify-es": {
-          "version": "3.3.9",
-          "resolved": "https://registry.npmjs.org/uglify-es/-/uglify-es-3.3.9.tgz",
-          "integrity": "sha512-r+MU0rfv4L/0eeW3xZrd16t4NZfK8Ld4SWVglYBb7ez5uXFWHuVRs6xCTrf1yirs9a4j4Y27nn7SRfO6v67XsQ==",
+        "source-map-support": {
+          "version": "0.5.12",
+          "resolved": "https://registry.npmjs.org/source-map-support/-/source-map-support-0.5.12.tgz",
+          "integrity": "sha512-4h2Pbvyy15EE02G+JOZpUCmqWJuqrs+sEkzewTm++BPi7Hvn/HwcqLAcNxYAyI0x13CpPPn+kMjl+hplXMHITQ==",
           "dev": true,
           "requires": {
-            "commander": "~2.13.0",
-            "source-map": "~0.6.1"
+            "buffer-from": "^1.0.0",
+            "source-map": "^0.6.0"
+          }
+        },
+        "terser": {
+          "version": "3.17.0",
+          "resolved": "https://registry.npmjs.org/terser/-/terser-3.17.0.tgz",
+          "integrity": "sha512-/FQzzPJmCpjAH9Xvk2paiWrFq+5M6aVOf+2KRbwhByISDX/EujxsK+BAvrhb6H+2rtrLCHK9N01wO014vrIwVQ==",
+          "dev": true,
+          "requires": {
+            "commander": "^2.19.0",
+            "source-map": "~0.6.1",
+            "source-map-support": "~0.5.10"
           }
         }
       }

package.json

@@ -175,7 +175,6 @@
     "addons-linter": "^1.10.0",
     "babel-core": "^6.26.3",
     "babel-eslint": "^8.0.0",
-    "babel-plugin-transform-async-to-generator": "^6.24.1",
     "babel-plugin-transform-class-properties": "^6.24.1",
     "babel-plugin-transform-object-rest-spread": "^6.22.0",
     "babel-plugin-transform-runtime": "^6.23.0",
@@ -223,7 +222,7 @@
     "gulp-sourcemaps": "^2.6.0",
     "gulp-stylefmt": "^1.1.0",
     "gulp-stylelint": "^7.0.0",
-    "gulp-uglify-es": "^1.0.1",
+    "gulp-uglify-es": "^1.0.4",
     "gulp-util": "^3.0.7",
     "gulp-watch": "^5.0.1",
     "gulp-zip": "^4.0.0",