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

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

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 
 import 'package:analysis_server/protocol/protocol_generated.dart';
 import 'package:analysis_server/src/computer/imported_elements_computer.dart';
 import 'package:analyzer/dart/analysis/results.dart';
 import 'package:test/test.dart';
@@ skipping 114 matching lines @@
     List<ImportedElements> elementsList = await _computeElements(
         content, content.indexOf(selection), selection.length);
     expect(elementsList, hasLength(1));
     ImportedElements elements = elementsList[0];
     expect(elements, isNotNull);
     expect(elements.path, '/lib/core/core.dart');
     expect(elements.prefix, 'core');
     expect(elements.elements, unorderedEquals(['String']));
   }
 
+  test_dartMath_noPrefix() async {
+    String selection = "new Random();";
+    String content = """
+import 'dart:math';
+bool randomBool() {
+  Random r = $selection
+  return r.nextBool();
+}
+""";
+    List<ImportedElements> elementsList = await _computeElements(
+        content, content.indexOf(selection), selection.length);
+    expect(elementsList, hasLength(1));
+    ImportedElements elements = elementsList[0];
+    expect(elements, isNotNull);
+    expect(elements.path, '/lib/math/math.dart');
+    expect(elements.prefix, '');
+    expect(elements.elements, unorderedEquals(['Random']));
+  }
+
   test_none_comment() async {
     String selection = 'comment';
     String content = """
 // Method $selection.
 blankLine() {
   print('');
 }
 """;
     List<ImportedElements> elementsList = await _computeElements(
         content, content.indexOf(selection), selection.length);
@@ skipping 182 matching lines @@
 
   Future<List<ImportedElements>> _computeElements(
       String sourceContent, int offset, int length) async {
     provider.newFile(sourcePath, sourceContent);
     ResolveResult result = await driver.getResult(sourcePath);
     ImportedElementsComputer computer =
         new ImportedElementsComputer(result.unit, offset, length);
     return computer.compute();
   }
 }