Remove Properties utility type and use Pick directly instead, because…

… Properties includes a class's methods, not just its properties, which is not desired, but cannot be worked around at this time, because TypeScript currently offers no way to form a union type of only those instance properties of a type which equal undefined on its prototype, presumably because a prototype is a value, not a type, and only types can be queried at compile time because that's the simple and straightforward way for it to work, in spite of the fact that the initial state of a class's prototype object at runtime *could* be statically calculated at compile time by analyzing the class's structure, if they *wanted* to, harumpf harumpf.

src/GameUpgrade.ts

@@ -1,6 +1,5 @@
 import {IGameState} from "./IGameState.ts";
 import {IGameUpgrade} from "./IGameUpgrade.ts";
-import {Properties} from "./util.ts";
 
 /**
  * See IGameUpgrade.
@@ -93,7 +92,15 @@ export class GameUpgrade implements IGameUpgrade {
      * the instance's properties, mimicking C/++'s designated initializers.
      * @param properties Keyword arguments / property donor object.
      */
-    public constructor(properties: Properties<GameUpgrade>) {
+    public constructor(properties: Pick<GameUpgrade,
+        'effectOrder' |
+        'speedFormulaEffect' |
+        'purchaseCondition' |
+        'purchaseAckMessage' |
+        'purchaseNakMessage' |
+        'postPurchaseEffect' |
+        'tickEffect'
+    >) {
         Object.assign(this, properties);
     }
 }

src/util.ts

@@ -1,9 +1,3 @@
-/**
- * Gets the type of a plain object that has the same properties
- * as an instance of the given class.
- */
-export type Properties<Klass> = Pick<Klass, keyof Klass>;
-
 /**
  * Applies an appropriate ordinal suffix to the given number
  * (i.e. "-st", "-nd", "-rd", or "-th").