Put tree-shaking behind a flag in dartk.

pkg/kernel/lib/target/vm.dart

Index: pkg/kernel/lib/target/vm.dart
diff --git a/pkg/kernel/lib/target/vm.dart b/pkg/kernel/lib/target/vm.dart
index d2643f5b5bdec5feaee1acb2cfc7e007806aa86b..8d4f84984614fc317537248fceb9ac9ab6d8efc6 100644
--- a/pkg/kernel/lib/target/vm.dart
+++ b/pkg/kernel/lib/target/vm.dart
@@ -4,6 +4,7 @@
 library kernel.target.vm;
 
 import '../ast.dart';
+import '../class_hierarchy.dart';
 import '../core_types.dart';
 import '../transformations/continuation.dart' as cont;
 import '../transformations/erasure.dart';
@@ -55,8 +56,7 @@ class VmTarget extends Target {
       ];
 
   void transformProgram(Program program) {
-    var mixins = new mix.MixinFullResolution();
-    mixins.transform(program);
+    var mixins = new mix.MixinFullResolution()..transform(program);
 
     var hierarchy = mixins.hierarchy;
     var coreTypes = new CoreTypes(program);
@@ -68,11 +68,13 @@ class VmTarget extends Target {
           .transformProgram(program);
     }
 
-    if (program.mainMethod != null) {
+    if (flags.treeShake) {
       new TreeShaker(program,
-              hierarchy: hierarchy, coreTypes: coreTypes,
+              hierarchy: hierarchy,
+              coreTypes: coreTypes,
               strongMode: strongMode)
           .transform(program);
+      hierarchy = null; // Hierarchy must be recomputed.
     }
 
     cont.transformProgram(program);