Add integration tests for postfix server api

pkg/analysis_server/test/integration/edit/is_postfix_completion_applicable_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 'package:analysis_server/protocol/protocol_generated.dart';
+import 'package:analyzer_plugin/protocol/protocol_common.dart';
+import 'package:test/test.dart';
+import 'package:test_reflective_loader/test_reflective_loader.dart';
+
+import '../support/integration_tests.dart';
+
+main() {
+  defineReflectiveSuite(() {
+    defineReflectiveTests(IsPostfixCompletionApplicableTest);
+  });
+}
+
+@reflectiveTest
+class IsPostfixCompletionApplicableTest
+    extends AbstractAnalysisServerIntegrationTest {
+  test_is_postfix_completion_applicable() async {
+    String pathname = sourcePath('test.dart');
+    String text = r'''
+void bar() {
+  foo();.tryon
+}
+void foo() { }
+''';
+    int loc = text.indexOf('.tryon');
+    text = text.replaceAll('.tryon', '');
+    writeFile(pathname, text);
+    standardAnalysisSetup();
+
+    await analysisFinished;
+    expect(currentAnalysisErrors[pathname], isEmpty);
+
+    // expect a postfix completion applicable result
+    EditIsPostfixCompletionApplicableResult result =
+        await sendEditIsPostfixCompletionApplicable(pathname, '.tryon', loc);
+    expect(result.value, isTrue);
+  }
+}