Added test case.

tests/cases/compiler/elaboratedErrorsOnNullableTargets01.ts

@@ -0,0 +1,7 @@
+
+export declare let x: null | { foo: { bar: string | null } | undefined } | undefined;
+export declare let y: { foo: { bar: number | undefined } };
+
+x = y;
+
+y = x;