add enableAsync to analysis option file flags

pkg/analysis_server/test/integration/analysis/error_test.dart

Index: pkg/analysis_server/test/integration/analysis/error_test.dart
diff --git a/pkg/analysis_server/test/integration/analysis/error_test.dart b/pkg/analysis_server/test/integration/analysis/error_test.dart
index cd490fa87e4e9c495ebae3296d1af5138f28fcee..be2f8c1d04f62820b4a3bbec38e5817113719e00 100644
--- a/pkg/analysis_server/test/integration/analysis/error_test.dart
+++ b/pkg/analysis_server/test/integration/analysis/error_test.dart
@@ -19,6 +19,45 @@ main() {
 @reflectiveTest
 class AnalysisErrorIntegrationTest
     extends AbstractAnalysisServerIntegrationTest {
+  test_async_disabled() async {
+    String pathname = sourcePath('test.dart');
+    writeFile(
+        pathname,
+        '''
+import 'dart:async';
+class Test {
+  Future foo() async {}
+}
+  ''');
+    await sendAnalysisUpdateOptions(new AnalysisOptions()..enableAsync = false);
+    standardAnalysisSetup();
+    await analysisFinished;
+    expect(currentAnalysisErrors[pathname], isList);
+    List<AnalysisError> errors = currentAnalysisErrors[pathname];
+    expect(errors, hasLength(1));
+    Set<String> allErrorMessages =
+        errors.map((AnalysisError e) => e.message).toSet();
+    expect(allErrorMessages, contains(
+      "Async and sync are not supported in this environment."));
+  }
+
+  test_async_enabled() async {
+    String pathname = sourcePath('test.dart');
+    writeFile(
+        pathname,
+        '''
+import 'dart:async';
+class Test {
+  Future foo() async {}
+}
+  ''');
+    standardAnalysisSetup();
+    await analysisFinished;
+    expect(currentAnalysisErrors[pathname], isList);
+    List<AnalysisError> errors = currentAnalysisErrors[pathname];
+    expect(errors, isEmpty);
+  }
+
   test_detect_simple_error() {
     String pathname = sourcePath('test.dart');
     writeFile(