Remove assert that prevents nested scopes closing over save variable

pkg/compiler/lib/src/js_model/closure_visitors.dart

Index: pkg/compiler/lib/src/js_model/closure_visitors.dart
diff --git a/pkg/compiler/lib/src/js_model/closure_visitors.dart b/pkg/compiler/lib/src/js_model/closure_visitors.dart
index a089defc3827a327e0927a4b884b3a6f1f22cc4b..1fdc580656d51fa651e63a26daf8ad8514a3b173 100644
--- a/pkg/compiler/lib/src/js_model/closure_visitors.dart
+++ b/pkg/compiler/lib/src/js_model/closure_visitors.dart
@@ -266,7 +266,6 @@ class CapturedScopeBuilder extends ir.Visitor {
         savedScopeInfo.freeVariables;
     assert(freeVariables.isEmpty || savedIsInsideClosure);
     for (ir.VariableDeclaration freeVariable in freeVariables) {
-      assert(!_capturedVariables.contains(freeVariable));
       _capturedVariables.add(freeVariable);
       _markVariableAsUsed(freeVariable);
     }