Support conditional directives in the options file.

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 9b96c6a9bf0f81db3236fd8d89bb76385720400a..e5989105aef0c14db2e1d3ae09b87c9ca1e96c2b 100644
--- a/pkg/analyzer/lib/src/task/options.dart
+++ b/pkg/analyzer/lib/src/task/options.dart
@@ -47,6 +47,8 @@ class AnalyzerOptions {
   static const String enableAsync = 'enableAsync';
   static const String enableGenericMethods = 'enableGenericMethods';
   static const String enableSuperMixins = 'enableSuperMixins';
+  static const String enableConditionalDirectives =
+      "enableConditionalDirectives";
   static const String errors = 'errors';
   static const String exclude = 'exclude';
   static const String language = 'language';
@@ -79,7 +81,8 @@ class AnalyzerOptions {
   static const List<String> languageOptions = const [
     enableAsync,
     enableGenericMethods,
-    enableSuperMixins
+    enableSuperMixins,
+    enableConditionalDirectives,
   ];
 }
 
@@ -490,6 +493,14 @@ class _OptionsProcessor {
         context.analysisOptions = options;
       }
     }
+    if (feature == AnalyzerOptions.enableConditionalDirectives) {
+      if (isTrue(value)) {
+        AnalysisOptionsImpl options =
+            new AnalysisOptionsImpl.from(context.analysisOptions);
+        options.enableConditionalDirectives = true;
+        context.analysisOptions = options;
+      }
+    }
   }
 
   void setLanguageOptions(AnalysisContext context, Object configs) {