Added test.

tests/cases/compiler/omitTypeHelperModifiers01.ts

@@ -0,0 +1,22 @@
+// @strict: true
+
+type A = {
+    a: number;
+    b?: string;
+    readonly c: boolean;
+    d: unknown;
+};
+
+type B = Omit<A, 'a'>;
+
+function f(x: B) {
+    const b = x.b;
+    x.b = "hello";
+    x.b = undefined;
+
+    const c = x.c;
+    x.c = true;
+
+    const d = x.d;
+    x.d = d;
+}