Enable async support

pkg/analyzer/lib/options.dart

Index: pkg/analyzer/lib/options.dart
diff --git a/pkg/analyzer/lib/options.dart b/pkg/analyzer/lib/options.dart
index 7dd0c178716de174d84c7d00a8b783038941920a..af3c712e9ac5f94c23d0cca9a2a52639ecdb56cb 100644
--- a/pkg/analyzer/lib/options.dart
+++ b/pkg/analyzer/lib/options.dart
@@ -26,9 +26,6 @@ class CommandLineOptions {
   /** Whether to display version information */
   final bool displayVersion;
 
-  /** Whether to enable support for the proposed async feature. */
-  final bool enableAsync;
-
   /** Whether to enable support for the proposed enum feature. */
   final bool enableEnum;
 
@@ -80,7 +77,6 @@ class CommandLineOptions {
         this.definedVariables = definedVariables,
         disableHints = args['no-hints'],
         displayVersion = args['version'],
-        enableAsync = args['enable-async'],
         enableEnum = args['enable-enum'],
         enableTypeChecks = args['enable_type_checks'],
         ignoreUnrecognizedFlags = args['ignore-unrecognized-flags'],