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

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 c12403ec6384ba44464b0e0c2c02e6587ffc298a..d2643f5b5bdec5feaee1acb2cfc7e007806aa86b 100644
--- a/pkg/kernel/lib/target/vm.dart
+++ b/pkg/kernel/lib/target/vm.dart
@@ -68,9 +68,12 @@ class VmTarget extends Target {
           .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);