The `resolve` failure (or `null` return) of import directive for
`java.lang.Thread.sleep` is legitimate, since there are two static methods
in `java.lang.Thread`:
https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Thread.html
hence not able to resolve to a single method.
Unless we change `UImportStatement` to be a subtype of `UMultiResolvable`,
instead of `UResolvable`, we can't properly test `resolve` API for that.
Jinseong Jeon
3769fd8e07