Use istanbul babel plugin only in tests (#536)

Prevents coverage instrumentation code from being
in the final package.

Fixes #535.

Author: Throne3d

.babelrc

@@ -9,7 +9,11 @@
       }
     ]
   ],
-  "plugins": [
-    "istanbul"
-  ]
+  "env": {
+    "test": {
+      "plugins": [
+        "istanbul"
+      ]
+    }
+  }
 }

package-lock.json

@@ -27,7 +27,7 @@
     "build": "babel lib --out-dir dist",
     "prepare": "npm run build",
     "lint": "eslint . --ignore-path .gitignore",
-    "coverage": "nyc --require @babel/register mocha -- 'test/*.test.js'",
+    "coverage": "cross-env NODE_ENV=test nyc --require @babel/register mocha -- 'test/*.test.js'",
     "report": "nyc report --reporter=text-lcov | coveralls",
     "test": "npm run lint && npm run coverage"
   },
@@ -57,6 +57,7 @@
     "babel-plugin-istanbul": "^5.2.0",
     "chai": "^4.2.0",
     "coveralls": "^3.0.9",
+    "cross-env": "^6.0.3",
     "eslint": "^6.7.2",
     "eslint-config-airbnb-base": "^14.0.0",
     "eslint-plugin-import": "^2.19.1",