Only tree-shake if there's a main program.

pkg/kernel/lib/target/vm.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.
 library kernel.target.vm;
 
 import '../ast.dart';
 import '../core_types.dart';
 import '../transformations/continuation.dart' as cont;
 import '../transformations/erasure.dart';
 import '../transformations/insert_covariance_checks.dart';
@@ skipping 50 matching lines @@
     var hierarchy = mixins.hierarchy;
     var coreTypes = new CoreTypes(program);
 
     if (strongMode) {
       new InsertTypeChecks(hierarchy: hierarchy, coreTypes: coreTypes)
           .transformProgram(program);
       new InsertCovarianceChecks(hierarchy: hierarchy, coreTypes: coreTypes)
           .transformProgram(program);
     }
 
-    new TreeShaker(program,
-            hierarchy: hierarchy, coreTypes: coreTypes, strongMode: strongMode)
-        .transform(program);
+    if (program.mainMethod != null) {
+      new TreeShaker(program,
+              hierarchy: hierarchy, coreTypes: coreTypes,
+              strongMode: strongMode)
+          .transform(program);
+    }
 
     cont.transformProgram(program);
 
     // Repair `_getMainClosure()` function in dart:_builtin.
     setup_builtin_library.transformProgram(program);
 
     if (strongMode) {
       new Erasure().transform(program);
     }
 
     new SanitizeForVM().transform(program);
   }
 }