dart2js: add support for configuration-specific imports.

pkg/compiler/lib/src/dart2js.dart

 // Copyright (c) 2013, 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.cmdline;
 
 import 'dart:async'
     show Future, EventSink;
 import 'dart:convert' show UTF8, LineSplitter;
 import 'dart:io'
@@ skipping 299 matching lines @@
         '--output-type=dart|--output-type=dart-multi|--output-type=js',
         setOutputType),
     new OptionHandler(Flags.useCpsIr, passThrough),
     new OptionHandler(Flags.noFrequencyBasedMinification, passThrough),
     new OptionHandler(Flags.verbose, setVerbose),
     new OptionHandler(Flags.version, (_) => wantVersion = true),
     new OptionHandler('--library-root=.+', setLibraryRoot),
     new OptionHandler('--out=.+|-o.*', setOutput, multipleArguments: true),
     new OptionHandler(Flags.allowMockCompilation, passThrough),
     new OptionHandler(Flags.fastStartup, passThrough),
+    new OptionHandler(Flags.conditionalDirectives, passThrough),
     new OptionHandler('${Flags.minify}|-m', implyCompilation),
     new OptionHandler(Flags.preserveUris, passThrough),
     new OptionHandler('--force-strip=.*', setStrip),
     new OptionHandler(Flags.disableDiagnosticColors,
                       (_) => diagnosticHandler.enableColors = false),
     new OptionHandler(Flags.enableDiagnosticColors,
                       (_) => diagnosticHandler.enableColors = true),
     new OptionHandler('--enable[_-]checked[_-]mode|--checked',
                       (_) => setCheckedMode(Flags.enableCheckedMode)),
     new OptionHandler(Flags.trustTypeAnnotations,
@@ skipping 436 matching lines @@
       } else if (exitCode == 253) {
         print(">>> TEST CRASH");
       } else {
         print(">>> TEST FAIL");
       }
       stderr.writeln(">>> EOF STDERR");
       subscription.resume();
     });
   });
 }