dart2js: add support for configuration-specific imports.

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

Index: pkg/compiler/lib/src/parser/diet_parser_task.dart
diff --git a/pkg/compiler/lib/src/parser/diet_parser_task.dart b/pkg/compiler/lib/src/parser/diet_parser_task.dart
index ee22ae949b0644626661a6347efb238ac5c6b26a..573d4b7000a860968bd98cad46b67fed4febf7c5 100644
--- a/pkg/compiler/lib/src/parser/diet_parser_task.dart
+++ b/pkg/compiler/lib/src/parser/diet_parser_task.dart
@@ -23,7 +23,12 @@ 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) {
@@ -31,7 +36,8 @@ class DietParserTask extends CompilerTask {
       Function idGenerator = compiler.getNextFreeClassId;
       ElementListener listener =
           new ElementListener(compiler, compilationUnit, idGenerator);
-      PartialParser parser = new PartialParser(listener);
+      PartialParser parser = new PartialParser(
+          listener, enableConditionalDirectives: _enableConditionalDirectives);
       try {
         parser.parseUnit(tokens);
       } on ParserError catch(_) {