fix errors and warnings in DDC

pkg/dev_compiler/lib/src/compiler/code_generator.dart

Index: pkg/dev_compiler/lib/src/compiler/code_generator.dart
diff --git a/pkg/dev_compiler/lib/src/compiler/code_generator.dart b/pkg/dev_compiler/lib/src/compiler/code_generator.dart
index e78370704bd1902abc7c36ada3a8ccfec639e81b..c86ccd4d3fbebece5f8623022109a29f758feac7 100644
--- a/pkg/dev_compiler/lib/src/compiler/code_generator.dart
+++ b/pkg/dev_compiler/lib/src/compiler/code_generator.dart
@@ -512,7 +512,7 @@ class CodeGenerator extends GeneralizingAstVisitor
       // We'll need to be consistent about when we're generating functions, and
       // only run this on the outermost function, and not any closures.
       inferNullableTypes(node);
-      return _visit(node);
+      return _visit(node) as JS.Node;
     });
 
     if (item != null) _moduleItems.add(item);