code: protected void ensureImportsChecked(List<JCCompilationUnit> trees) {
          // if there remain any unimported toplevels (these must have
          // no classes at all), process their import statements as well.
          for (JCCompilationUnit tree : trees) {
              if (!tree.starImportScope.isFilled()) {
                  Env<AttrContext> topEnv = enter.topLevelEnv(tree);
                  finishImports(tree, () -> { completeClass.resolveImports(tree, topEnv); });
              }
          }
      }
docs: /* Verify Imports: