fixes #197, don't generate casts that always pass

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 8a769573489637c8247f625e89d4fc13b699bf65..4f9878a4fcec5d80451788ed1acbd417f027b36d 100644
--- a/lib/src/codegen/js_codegen.dart
+++ b/lib/src/codegen/js_codegen.dart
@@ -253,6 +253,9 @@ class JSCodegenVisitor extends GeneralizingAstVisitor {
     var from = getStaticType(node.expression);
     var to = node.type.type;
 
+    // Skip the cast if it's not needed.
+    if (rules.isSubTypeOf(from, to)) return _visit(node.expression);
+
     // All Dart number types map to a JS double.
     if (rules.isNumType(from) &&
         (rules.isIntType(to) || rules.isDoubleType(to))) {