refactor code completion to allow for multiple suggestion computers

pkg/analysis_services/test/completion/completion_computer_test.dart

+// Copyright (c) 2014, 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.
+
+library test.services.completion.suggestion;
+
+import 'package:analysis_services/completion/completion_computer.dart';
+import 'package:analysis_services/src/completion/top_level_computer.dart';
+import 'package:analysis_testing/abstract_single_unit.dart';
+import 'package:analysis_testing/reflective_tests.dart';
+import 'package:unittest/unittest.dart';
+
+main() {
+  groupSep = ' | ';
+  runReflectiveTests(CompletionComputerTest);
+}
+
+@ReflectiveTestCase()
+class CompletionComputerTest extends AbstractSingleUnitTest {
+
+  test_topLevel() {
+    CompletionComputer.create(null).then((computers) {
+      assertContainsType(computers, TopLevelComputer);
+      expect(computers, hasLength(1));
+    });
+  }
+
+  /// Assert that the list contains exactly one of the given type
+  void assertContainsType(List computers, Type type) {
+    int count = 0;
+    computers.forEach((c) {
+      if (c.runtimeType == type) {
+        ++count;
+      }
+    });
+    if (count != 1) {
+      var msg = new StringBuffer();
+      msg.writeln('Expected $type, but found:');
+      computers.forEach((c) {
+        msg.writeln('  ${c.runtimeType}');
+      });
+      fail(msg.toString());
+    }
+  }
+}