Restore code coverage in Karma config

The transpiling step is done by babel instead of bublé now.
Bublé caused the istanbul instrumenter step to crash because of an
unexpected SourceMap object format.

package.json

@@ -41,12 +41,15 @@
     "rbush": "2.0.2"
   },
   "devDependencies": {
+    "@babel/core": "^7.4.0",
+    "@babel/preset-env": "^7.4.2",
     "@openlayers/eslint-plugin": "^4.0.0-beta.2",
     "@types/arcgis-rest-api": "^10.4.4",
     "@types/geojson": "^7946.0.7",
     "@types/pbf": "^3.0.1",
     "@types/rbush": "^2.0.2",
     "@types/topojson-specification": "^1.0.1",
+    "babel-loader": "^8.0.5",
     "buble": "^0.19.7",
     "buble-loader": "^0.5.1",
     "chaikin-smooth": "^1.0.4",
@@ -63,12 +66,14 @@
     "handlebars": "4.1.2",
     "html-to-image": "^0.1.0",
     "istanbul": "0.4.5",
+    "istanbul-instrumenter-loader": "^3.0.1",
     "jquery": "3.4.1",
     "jsdoc": "3.5.5",
     "jsdoc-plugin-typescript": "^1.0.8",
     "karma": "^4.0.1",
     "karma-chrome-launcher": "2.2.0",
     "karma-coverage": "^1.1.2",
+    "karma-coverage-istanbul-reporter": "^2.0.5",
     "karma-firefox-launcher": "^1.1.0",
     "karma-mocha": "1.3.0",
     "karma-sourcemap-loader": "^0.3.7",