Put tree-shaking behind a flag in dartk.

pkg/kernel/bin/dartk.dart

Index: pkg/kernel/bin/dartk.dart
diff --git a/pkg/kernel/bin/dartk.dart b/pkg/kernel/bin/dartk.dart
index b8666a43c45b26b7c20d994cfb14075b23e8981e..5a9ab99c39033048d541a19e7a33123646e81df7 100755
--- a/pkg/kernel/bin/dartk.dart
+++ b/pkg/kernel/bin/dartk.dart
@@ -77,7 +77,9 @@ ArgParser parser = new ArgParser(allowTrailingOptions: true)
       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
@@ -288,7 +290,8 @@ Future<CompilerOutcome> batchMain(
   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>{};
@@ -400,5 +403,9 @@ Future<CompilerOutcome> batchMain(
     print('writer.flush_time = $flushTime ms');
   }
 
+  if (options['tolerant']) {
+    return CompilerOutcome.Ok;
+  }
+
   return errors.length > 0 ? CompilerOutcome.Fail : CompilerOutcome.Ok;
 }