Add tests

Author: ahejlsberg

tests/cases/conformance/types/mapped/mappedTypesAndObjects.ts

@@ -0,0 +1,35 @@
+// @strictNullChecks: true
+// @declaration: true
+
+function f1<T>(x: Partial<T>, y: Readonly<T>) {
+    let obj: {};
+    obj = x;
+    obj = y;
+}
+
+function f2<T>(x: Partial<T>, y: Readonly<T>) {
+    let obj: { [x: string]: any };
+    obj = x;
+    obj = y;
+}
+
+// Repro from #12900
+
+interface Base {
+   foo: { [key: string]: any };
+   bar: any;
+   baz: any;
+}
+
+interface E1<T> extends Base {
+  foo: T;
+}
+
+interface Something { name: string, value: string };
+interface E2 extends Base {
+  foo: Partial<Something>;  // or other mapped type
+}
+
+interface E3<T> extends Base {
+  foo: Partial<T>; // or other mapped type
+}