set loose property assignments to computed if it's a literal

lib/babel/transformation/transformers/es6/properties.computed.js

@@ -51,7 +51,7 @@ var loose = function (node, body, objId) {
     body.push(t.expressionStatement(
       t.assignmentExpression(
         "=",
-        t.memberExpression(objId, prop.key, prop.computed),
+        t.memberExpression(objId, prop.key, prop.computed || t.isLiteral(prop.key)),
         prop.value
       )
     ));