Actually make the test file use '--strict'.

Author: DanielRosenwasser

tests/cases/compiler/elaboratedErrorsOnNullableTargets01.ts

@@ -1,7 +1,8 @@
+// @strict: true
 
 export declare let x: null | { foo: { bar: string | null } | undefined } | undefined;
 export declare let y: { foo: { bar: number | undefined } };
 
 x = y;
 
-y = x;
+y = x;