Allow S->T <: dynamic->T

lib/src/codegen/js_codegen.dart

Index: lib/src/codegen/js_codegen.dart
diff --git a/lib/src/codegen/js_codegen.dart b/lib/src/codegen/js_codegen.dart
index c705287d6caa5c606de964b28dfa5bf4cd5f1b24..1c63fef439a6fadd7d1a6a3a45f3c5a03b0baa2e 100644
--- a/lib/src/codegen/js_codegen.dart
+++ b/lib/src/codegen/js_codegen.dart
@@ -179,6 +179,16 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ConversionVisitor {
     return _emitCast(node.expression, to);
   }
 
+  // TODO(vsm): This should go away in the future.  I'm passing in the type
+  // as a String for now to avoid breaking any existing code.  In most cases,
+  // we currently throw for function types.
+  @override
+  visitClosureWrapBase(ClosureWrapBase node) {
+    var expression = _visit(node.expression);
+    var typeString = js.escapedString(node.convertedType.toString());
+    return js.call('dart.closureWrap(#, #)', [expression, typeString]);
+  }
+
   @override
   visitAsExpression(AsExpression node) =>
       _emitCast(node.expression, node.type.type);