Compute [WorldImpact] for main method as entrypoint

pkg/compiler/lib/src/js_backend/backend_impact.dart

Index: pkg/compiler/lib/src/js_backend/backend_impact.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend_impact.dart b/pkg/compiler/lib/src/js_backend/backend_impact.dart
index 7726ed0df65821b2e9ab49eb9007987579cebb64..afbd270c88f853987f1f028258b68b06cd344413 100644
--- a/pkg/compiler/lib/src/js_backend/backend_impact.dart
+++ b/pkg/compiler/lib/src/js_backend/backend_impact.dart
@@ -83,6 +83,18 @@ class BackendImpacts {
     return _computeSignature;
   }
 
+  BackendImpact _mainWithArguments;
+
+  BackendImpact get mainWithArguments {
+    if (_mainWithArguments == null) {
+      _mainWithArguments = new BackendImpact(
+          instantiatedClasses: [
+            helpers.jsArrayClass,
+            helpers.jsStringClass]);
+    }
+    return _mainWithArguments;
+  }
+
   BackendImpact _asyncBody;
 
   BackendImpact get asyncBody {