Repeats and fixes the changes landed & reverted as CL 1789553003.

pkg/compiler/lib/src/parser/parser_task.dart

Index: pkg/compiler/lib/src/parser/parser_task.dart
diff --git a/pkg/compiler/lib/src/parser/parser_task.dart b/pkg/compiler/lib/src/parser/parser_task.dart
index fa715aac5b537b4bcba50448ad3312198a88533c..a79875fba4a36005530a03932c0e11fb5187f5c9 100644
--- a/pkg/compiler/lib/src/parser/parser_task.dart
+++ b/pkg/compiler/lib/src/parser/parser_task.dart
@@ -11,6 +11,8 @@ import '../compiler.dart' show
     Compiler;
 import '../elements/modelx.dart' show
     ElementX;
+import '../options.dart' show
+    ParserOptions;
 import '../tokens/token.dart' show
     Token;
 import '../tree/tree.dart' show
@@ -26,12 +28,9 @@ import 'parser.dart' show
     Parser;
 
 class ParserTask extends CompilerTask {
-  final bool _enableConditionalDirectives;
+  final ParserOptions parserOptions;
 
-  ParserTask(Compiler compiler,
-             {bool enableConditionalDirectives: false})
-      : this._enableConditionalDirectives = enableConditionalDirectives,
-        super(compiler);
+  ParserTask(Compiler compiler, this.parserOptions) : super(compiler);
 
   String get name => 'Parser';
 
@@ -43,8 +42,7 @@ class ParserTask extends CompilerTask {
     return measure(() {
       NodeListener listener = new NodeListener(
           const ScannerOptions(), reporter, null);
-      Parser parser = new Parser(
-          listener, enableConditionalDirectives: _enableConditionalDirectives);
+      Parser parser = new Parser(listener, parserOptions);
       try {
         parser.parseUnit(token);
       } on ParserError catch(_) {