dart2js: add support for configuration-specific imports.

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

 // Copyright (c) 2015, 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.diet.task;
 
 import '../common.dart';
 import '../common/tasks.dart' show
     CompilerTask;
 import '../compiler.dart' show
     Compiler;
 import '../elements/elements.dart' show
     CompilationUnitElement;
 import '../tokens/token.dart' show
     Token;
 
 import 'listener.dart' show
     ParserError;
 import 'element_listener.dart' show
     ElementListener,
     ScannerOptions;
 import 'partial_parser.dart' show
     PartialParser;
 
 class DietParserTask extends CompilerTask {
-  DietParserTask(Compiler compiler) : super(compiler);
+  final bool _enableConditionalDirectives;
+
+  DietParserTask(Compiler compiler, {bool enableConditionalDirectives})
+      : this._enableConditionalDirectives = enableConditionalDirectives,
+        super(compiler);
+
   final String name = 'Diet Parser';
 
   dietParse(CompilationUnitElement compilationUnit, Token tokens) {
     measure(() {
       Function idGenerator = compiler.getNextFreeClassId;
       ScannerOptions scannerOptions = new ScannerOptions(
           canUseNative: compiler.backend.canLibraryUseNative(
               compilationUnit.library));
       ElementListener listener = new ElementListener(
           scannerOptions, compiler.reporter, compilationUnit, idGenerator);
-      PartialParser parser = new PartialParser(listener);
+      PartialParser parser = new PartialParser(
+          listener, enableConditionalDirectives: _enableConditionalDirectives);
       try {
         parser.parseUnit(tokens);
       } on ParserError catch(_) {
         assert(invariant(compilationUnit, compiler.compilationFailed));
       }
     });
   }
 }