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 10a0a9f7df255b9414b206ddac8b7575f63d5763..9afc8d4dcb90e6f532ce029a5d19aaad3aaa18f7 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) {
@@ -34,7 +39,8 @@ class DietParserTask extends CompilerTask {
               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(_) {