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

tests/language/if_null_assignment_behavior_test.dart

Index: tests/language/if_null_assignment_behavior_test.dart
diff --git a/tests/language/if_null_assignment_behavior_test.dart b/tests/language/if_null_assignment_behavior_test.dart
index 548d94ace8e8b8a5973d4c278db1a171813befe7..ca2ce93ec722b97a96e94a36585920fb83f225f1 100644
--- a/tests/language/if_null_assignment_behavior_test.dart
+++ b/tests/language/if_null_assignment_behavior_test.dart
@@ -170,6 +170,8 @@ main() {
   { final l = 1; check(1, () => l ??= bad(), []); } /// 13: static type warning
   { final l = null; yGetValue = 1; checkThrows(noMethod, () => l ??= y, ['y']); } /// 14: static type warning
   check(C, () => C ??= bad(), []); /// 15: static type warning
+  h ??= null; /// 29: compile-time error
+  h[0] ??= null; /// 30: compile-time error
 
   // C.v ??= e is equivalent to ((x) => x == null ? C.v = e : x)(C.v)
   C.xGetValue = 1; check(1, () => C.x ??= bad(), ['C.x']); /// 16: ok