extensionUtils: Add InjectionManager

It is fairly common for extensions to monkey-patch existing
classes. Add a small helper class that makes this a tad bit
more convenient.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2809>

tests/unit/injectionManager.js

@@ -0,0 +1,93 @@
+const JsUnit = imports.jsUnit;
+
+import '../../js/ui/environment.js';
+
+import {InjectionManager} from '../../js/extensions/extension.js';
+
+class Object1 {
+    count = 0;
+
+    getNumber() {
+        return 42;
+    }
+
+    getCount() {
+        return ++this.count;
+    }
+}
+
+/**
+ * @param {object} object to modify
+ */
+function addInjections(object) {
+    // extend original method
+    injectionManager.overrideMethod(
+        object, 'getNumber', originalMethod => {
+            return function () {
+                // eslint-disable-next-line no-invalid-this
+                const num = originalMethod.call(this);
+                return 2 * num;
+            };
+        });
+
+    // override original method
+    injectionManager.overrideMethod(
+        object, 'getCount', () => {
+            return function () {
+                return 42;
+            };
+        });
+
+    // inject new method
+    injectionManager.overrideMethod(
+        object, 'getOtherNumber', () => {
+            return function () {
+                return 42;
+            };
+        });
+}
+
+
+const injectionManager = new InjectionManager();
+let obj;
+
+// Prototype injections
+addInjections(Object1.prototype);
+
+obj = new Object1();
+
+// new obj is modified
+JsUnit.assertEquals(obj.getNumber(), 84);
+JsUnit.assertEquals(obj.getCount(), 42);
+JsUnit.assertEquals(obj.count, 0);
+JsUnit.assertEquals(obj.getOtherNumber(), 42);
+
+injectionManager.clear();
+
+obj = new Object1();
+
+// new obj is unmodified
+JsUnit.assertEquals(obj.getNumber(), 42);
+JsUnit.assertEquals(obj.getCount(), obj.count);
+JsUnit.assert(obj.count > 0);
+JsUnit.assertRaises(() => obj.getOtherNumber());
+
+// instance injections
+addInjections(obj);
+
+// obj is now modified
+JsUnit.assertEquals(obj.getNumber(), 84);
+JsUnit.assertEquals(obj.getCount(), 42);
+JsUnit.assertEquals(obj.count, 1);
+JsUnit.assertEquals(obj.getOtherNumber(), 42);
+
+injectionManager.restoreMethod(obj, 'getNumber');
+JsUnit.assertEquals(obj.getNumber(), 42);
+
+injectionManager.clear();
+
+// obj is unmodified again
+JsUnit.assertEquals(obj.getNumber(), 42);
+JsUnit.assertEquals(obj.getCount(), obj.count);
+JsUnit.assert(obj.count > 0);
+JsUnit.assertRaises(() => obj.getOtherNumber());