Fix: Use ts utilities function to determine variable type (#136)

This fixes issue an where async function will mark all
variable declarations as constant.

Author: soda0289

lib/ast-converter.js

@@ -899,8 +899,10 @@ module.exports = function(ast, extra) {
 
                 var varStatementKind;
 
-                if (node.declarationList.flags) {
-                    varStatementKind = (node.declarationList.flags === ts.NodeFlags.Let) ? "let" : "const";
+                if (ts.isLet(node.declarationList)) {
+                    varStatementKind = "let";
+                } else if (ts.isConst(node.declarationList)) {
+                    varStatementKind = "const";
                 } else {
                     varStatementKind = "var";
                 }