Fix analyzer's handling of import prefixes not followed by '.'.

tests/language/conditional_property_assignment_test.dart

Index: tests/language/conditional_property_assignment_test.dart
diff --git a/tests/language/conditional_property_assignment_test.dart b/tests/language/conditional_property_assignment_test.dart
index bf77842968a9414df7bdea51521b1ccbd6fcd087..6b348216a7f75daccebe440266dbdf32ab4823ee 100644
--- a/tests/language/conditional_property_assignment_test.dart
+++ b/tests/language/conditional_property_assignment_test.dart
@@ -87,7 +87,7 @@ main() {
 
   // Nor can it be used to assign to toplevel properties in libraries imported
   // via prefix.
-  Expect.throws(() => h?.topLevelVar = null, noMethod); /// 20: static type warning
-  Expect.throws(() => h?.topLevelVar += null, noMethod); /// 21: static type warning
-  Expect.throws(() => h?.topLevelVar ??= null, noMethod); /// 22: static type warning
+  h?.topLevelVar = null; /// 20: compile-time error
+  h?.topLevelVar += null; /// 21: compile-time error
+  h?.topLevelVar ??= null; /// 22: compile-time error
 }