dart2js: add support for configuration-specific imports.

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

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.parser.task;
 
 import '../common.dart';
 import '../common/tasks.dart' show
     CompilerTask;
 import '../compiler.dart' show
     Compiler;
 import '../elements/modelx.dart' show
     ElementX;
 import '../tokens/token.dart' show
     Token;
 import '../tree/tree.dart' show
     Node;
 
 import 'element_listener.dart' show
     ScannerOptions;
 import 'listener.dart' show
     ParserError;
 import 'node_listener.dart' show
     NodeListener;
 import 'parser.dart' show
     Parser;
 
 class ParserTask extends CompilerTask {
-  ParserTask(Compiler compiler) : super(compiler);
+  final bool _enableConditionalDirectives;
+
+  ParserTask(Compiler compiler,
+             {bool enableConditionalDirectives: false})
+      : this._enableConditionalDirectives = enableConditionalDirectives,
+        super(compiler);
+
   String get name => 'Parser';
 
   Node parse(ElementX element) {
     return measure(() => element.parseNode(compiler.parsing));
   }
 
   Node parseCompilationUnit(Token token) {
     return measure(() {
       NodeListener listener = new NodeListener(
           const ScannerOptions(), reporter, null);
-      Parser parser = new Parser(listener);
+      Parser parser = new Parser(
+          listener, enableConditionalDirectives: _enableConditionalDirectives);
       try {
         parser.parseUnit(token);
       } on ParserError catch(_) {
         assert(invariant(token, compiler.compilationFailed));
         return listener.makeNodeList(0, null, null, '\n');
       }
       Node result = listener.popNode();
       assert(listener.nodes.isEmpty);
       return result;
     });
   }
 }