Fix insertion of imports when there are no existing directives (issue 30430)

pkg/analysis_server/test/src/computer/import_elements_computer_test.dart

Index: pkg/analysis_server/test/src/computer/import_elements_computer_test.dart
diff --git a/pkg/analysis_server/test/src/computer/import_elements_computer_test.dart b/pkg/analysis_server/test/src/computer/import_elements_computer_test.dart
index 3dc4631115da56de53235751de09e309f12e2c9f..63d54320ca920119ae47575ed16ad8be6041fc42 100644
--- a/pkg/analysis_server/test/src/computer/import_elements_computer_test.dart
+++ b/pkg/analysis_server/test/src/computer/import_elements_computer_test.dart
@@ -58,6 +58,24 @@ class ImportElementsComputerTest extends AbstractContextTest {
     path = provider.convertPath('/test.dart');
   }
 
+  test_createEdits_addImport_noDirectives() async {
+    await createBuilder('''
+main() {
+  // paste here
+}
+''');
+    await computeChanges(<ImportedElements>[
+      new ImportedElements('/lib/math/math.dart', '', <String>['Random'])
+    ]);
+    assertChanges('''
+import 'dart:math';
+
+main() {
+  // paste here
+}
+''');
+  }
+
   test_createEdits_addImport_noPrefix() async {
     Source fooSource = addPackageSource('pkg', 'foo.dart', '');
     await createBuilder('''