add parser parseAsync flag

pkg/analyzer/lib/src/task/options.dart

Index: pkg/analyzer/lib/src/task/options.dart
diff --git a/pkg/analyzer/lib/src/task/options.dart b/pkg/analyzer/lib/src/task/options.dart
index ebc5a4c2344cbc35fa9d6734d5ac16f120b6bcbf..6775408ba659b07e5ccbc77818d29682727c5271 100644
--- a/pkg/analyzer/lib/src/task/options.dart
+++ b/pkg/analyzer/lib/src/task/options.dart
@@ -39,6 +39,7 @@ void configureContextOptions(
 /// `analyzer` analysis options constants.
 class AnalyzerOptions {
   static const String analyzer = 'analyzer';
+  static const String enableAsync = 'enableAsync';
   static const String enableGenericMethods = 'enableGenericMethods';
   static const String enableSuperMixins = 'enableSuperMixins';
   static const String errors = 'errors';
@@ -71,6 +72,7 @@ class AnalyzerOptions {
 
   /// Supported `analyzer` language configuration options.
   static const List<String> languageOptions = const [
+    enableAsync,
     enableGenericMethods,
     enableSuperMixins
   ];
@@ -425,6 +427,14 @@ class _OptionsProcessor {
 
   void setLanguageOption(
       AnalysisContext context, Object feature, Object value) {
+    if (feature == AnalyzerOptions.enableAsync) {
+      if (isFalse(value)) {
+        AnalysisOptionsImpl options =
+            new AnalysisOptionsImpl.from(context.analysisOptions);

[Brian Wilkerson, 2016/01/08 18:43:47]

Seems kind of wasteful to create a new options object for every flag. It might
not matter much in terms of performance, though.

[danrubel, 2016/01/08 20:45:51]

I don't think this is a performance bottleneck, so I did not spend the time to
optimize the performance.

+        options.enableAsync = false;
+        context.analysisOptions = options;
+      }
+    }
     if (feature == AnalyzerOptions.enableSuperMixins) {
       if (isTrue(value)) {
         AnalysisOptionsImpl options =