generator: add isLast helper method

lib/6to5/generator.js

@@ -70,7 +70,7 @@ CodeGenerator.prototype.push = function (str) {
     str = str.replace(/\n/g, "\n" + indent);
 
     // we've got a newline before us so prepend on the indentation
-    if (_.last(this.buf) === "\n") str = indent + str;
+    if (this.isLast("\n")) str = indent + str;
   }
 
   var self = this;
@@ -87,6 +87,10 @@ CodeGenerator.prototype.push = function (str) {
   this.buf += str;
 };
 
+CodeGenerator.prototype.isLast = function (cha) {
+  return _.last(this.buf) === cha;
+};
+
 CodeGenerator.prototype.getIndent = function () {
   return util.repeat(this._indent * this.tabWidth);
 };