dart2js: add support for configuration-specific imports.

pkg/compiler/lib/src/apiimpl.dart

 // Copyright (c) 2012, 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 leg_apiimpl;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'package:package_config/packages.dart';
@@ skipping 90 matching lines @@
             deferredMapUri: extractUriOption(options, '--deferred-map='),
             dumpInfo: hasOption(options, Flags.dumpInfo),
             buildId: extractStringOption(
                 options, '--build-id=',
                 "build number could not be determined"),
             showPackageWarnings:
                 hasOption(options, Flags.showPackageWarnings),
             useContentSecurityPolicy:
               hasOption(options, Flags.useContentSecurityPolicy),
             useStartupEmitter: hasOption(options, Flags.fastStartup),
+            enableConditionalDirectives:
+                hasOption(options, Flags.conditionalDirectives),
             hasIncrementalSupport:
                 forceIncrementalSupport ||
                 hasOption(options, Flags.incrementalSupport),
             suppressWarnings: hasOption(options, Flags.suppressWarnings),
             fatalWarnings: hasOption(options, Flags.fatalWarnings),
             enableExperimentalMirrors:
                 hasOption(options, Flags.enableExperimentalMirrors),
             enableAssertMessage:
                 hasOption(options, Flags.enableAssertMessage),
             generateCodeWithCompileTimeErrors:
@@ skipping 423 matching lines @@
     print('$message: ${tryToString(exception)}');
     print(tryToString(stackTrace));
   }
 
   fromEnvironment(String name) => environment[name];
 
   LibraryInfo lookupLibraryInfo(String libraryName) {
     return library_info.LIBRARIES[libraryName];
   }
 }