Make "weak" optional dependency and check it at runtime (#4984)

* Make "weak" optional dependency and check it at runtime

* Update index.js

Author: mjesun

packages/jest-leak-detector/package.json

@@ -8,7 +8,9 @@
   "license": "MIT",
   "main": "build/index.js",
   "dependencies": {
-    "pretty-format": "^21.2.1",
+    "pretty-format": "^21.2.1"
+  },
+  "optionalDependencies": {
     "weak": "^1.0.1"
   }
 }

packages/jest-leak-detector/src/index.js

@@ -12,7 +12,6 @@
 import prettyFormat from 'pretty-format';
 import v8 from 'v8';
 import vm from 'vm';
-import weak from 'weak';
 
 const PRIMITIVE_TYPES = new Set([
   'undefined',
@@ -35,6 +34,17 @@ export default class {
       );
     }
 
+    let weak;
+
+    try {
+      weak = require('weak');
+    } catch (err) {
+      throw new Error(
+        'The leaking detection mechanism requires the "weak" package to work. ' +
+          'Please make sure that you can install the native dependency on your platform.',
+      );
+    }
+
     weak(value, () => (this._isReferenceBeingHeld = false));
     this._isReferenceBeingHeld = true;