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 98 matching lines @@
             sourceMapUri: extractUriOption(options, '--source-map='),
             outputUri: extractUriOption(options, '--out='),
             deferredMapUri: extractUriOption(options, '--deferred-map='),
             dumpInfo: hasOption(options, Flags.dumpInfo),
             buildId: extractStringOption(
                 options, '--build-id=',
                 "build number could not be determined"),
             useContentSecurityPolicy:
               hasOption(options, Flags.useContentSecurityPolicy),
             useStartupEmitter: hasOption(options, Flags.fastStartup),
+            enableConditionalDirectives:
+                hasOption(options, Flags.conditionalDirectives),
             hasIncrementalSupport:
                 forceIncrementalSupport ||
                 hasOption(options, Flags.incrementalSupport),
             diagnosticOptions: new DiagnosticOptions(
                 suppressWarnings: hasOption(options, Flags.suppressWarnings),
                 fatalWarnings: hasOption(options, Flags.fatalWarnings),
                 suppressHints: hasOption(options, Flags.suppressHints),
                 terseDiagnostics: hasOption(options, Flags.terse),
                 showPackageWarnings:
                     hasOption(options, Flags.showPackageWarnings)),
@@ skipping 434 matching lines @@
   Uri lookupLibraryUri(String libraryName) {
     assert(invariant(NO_LOCATION_SPANNABLE,
         sdkLibraries != null, message: "setupSdk() has not been run"));
     return sdkLibraries[libraryName];
   }
 
   Uri resolvePatchUri(String libraryName) {
     return backend.resolvePatchUri(libraryName, platformConfigUri);
   }
 }