Fix: Use ts utilities determine variable declaration type (fixes #136)

This fixes an issue where async functions will mark all
variable declarations as constant. We change the way node flags
are read by using ts utilities functions (isLet and isConst)

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";
                 }