Put tree-shaking behind a flag in dartk.

pkg/kernel/lib/target/targets.dart

Index: pkg/kernel/lib/target/targets.dart
diff --git a/pkg/kernel/lib/target/targets.dart b/pkg/kernel/lib/target/targets.dart
index 1c0627311f60c3836db170bf356d1d6c181cd927..b2380367d95580c362d0650185cc40db5474e5d8 100644
--- a/pkg/kernel/lib/target/targets.dart
+++ b/pkg/kernel/lib/target/targets.dart
@@ -13,7 +13,9 @@ final List<String> targetNames = targets.keys.toList();
 
 class TargetFlags {
   bool strongMode;
-  TargetFlags({this.strongMode: false});
+  bool treeShake;
+
+  TargetFlags({this.strongMode: false, this.treeShake: false});
 }
 
 typedef Target _TargetBuilder(TargetFlags flags);