Put tree-shaking behind a flag in dartk.

pkg/kernel/bin/dartk.dart

 #!/usr/bin/env dart
 // Copyright (c) 2016, 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.
 
 import 'dart:async';
 import 'dart:io';
 
 import 'batch_util.dart';
 
@@ skipping 59 matching lines @@
       defaultsTo: false)
   ..addOption('D',
       abbr: 'D',
       allowMultiple: true,
       help: 'Define an environment variable.',
       hide: true)
   ..addFlag('show-external',
       help: 'When printing a library as text, also print its dependencies\n'
           'on external libraries.')
   ..addFlag('show-offsets',
-      help: 'When printing a library as text, also print node offsets');
+      help: 'When printing a library as text, also print node offsets')
+  ..addFlag('tree-shake',
+      defaultsTo: false, help: 'Enable tree-shaking if the target supports it');
 
 String getUsage() => """
 Usage: dartk [options] FILE
 
 Convert .dart or .dill files to kernel's IR and print out its textual
 or binary form.
 
 Examples:
     dartk foo.dart            # print text IR for foo.dart
     dartk foo.dart -ofoo.dill # write binary IR for foo.dart to foo.dill
@@ skipping 190 matching lines @@
   List<String> urlMapping = options['url-mapping'] as List<String>;
   var customUriMappings = parseCustomUriMappings(urlMapping);
   var repository = new Repository();
 
   Program program;
 
   var watch = new Stopwatch()..start();
   List<String> loadedFiles;
   Function getLoadedFiles;
   List errors = const [];
-  TargetFlags targetFlags = new TargetFlags(strongMode: options['strong']);
+  TargetFlags targetFlags = new TargetFlags(
+      strongMode: options['strong'], treeShake: options['tree-shake']);
   Target target = getTarget(options['target'], targetFlags);
 
   var declaredVariables = <String, String>{};
   declaredVariables.addAll(target.extraDeclaredVariables);
   for (String define in options['D']) {
     int separator = define.indexOf('=');
     if (separator == -1) {
       fail('Invalid define: -D$define. Format is -D<name>=<value>');
     }
     String name = define.substring(0, separator);
@@ skipping 91 matching lines @@
     print('writer.time = $time ms');
   }
 
   await ioFuture;
 
   if (shouldReportMetrics) {
     int flushTime = watch.elapsedMilliseconds - time;
     print('writer.flush_time = $flushTime ms');
   }
 
+  if (options['tolerant']) {
+    return CompilerOutcome.Ok;
+  }
+
   return errors.length > 0 ? CompilerOutcome.Fail : CompilerOutcome.Ok;
 }