Avoid using Object.isFrozen to prevent dev/prod differences.

If you revert the previous commit and run `npm test`, you'll see all the
tests this dynamic Object.isFrozen check has been silently protecting
from failing, but only (and this is the important part) in development.

Since we only actually freeze objects with Object.freeze in development,
this Object.isFrozen check does not help in production, so an object
that would have been frozen in development gets reused as a mutable
copy, potentially acquiring properties it should not acquire (a bug
fixed by the previous commit, first reported in issue #9735).

src/utilities/common/mergeDeep.ts

@@ -101,21 +101,17 @@ export class DeepMerger<TContextArgs extends any[]> {
 
   public shallowCopyForMerge<T>(value: T): T {
     if (isNonNullObject(value)) {
-      if (this.pastCopies.has(value)) {
-        // In order to reuse a past copy, it must be mutable, but copied objects
-        // can sometimes be frozen while this DeepMerger is still active.
-        if (!Object.isFrozen(value)) return value;
-        this.pastCopies.delete(value);
-      }
-      if (Array.isArray(value)) {
-        value = (value as any).slice(0);
-      } else {
-        value = {
-          __proto__: Object.getPrototypeOf(value),
-          ...value,
-        };
+      if (!this.pastCopies.has(value)) {
+        if (Array.isArray(value)) {
+          value = (value as any).slice(0);
+        } else {
+          value = {
+            __proto__: Object.getPrototypeOf(value),
+            ...value,
+          };
+        }
+        this.pastCopies.add(value);
       }
-      this.pastCopies.add(value);
     }
     return value;
   }