add back _property-literals transformer that somehow disappeared?

lib/6to5/transformation/transformers/_property-literals.js

@@ -0,0 +1,14 @@
+var esutils = require("esutils");
+var t       = require("../../types");
+
+exports.Property = function (node) {
+  // ignore key literals that are valid identifiers
+  var key = node.key;
+  if (t.isLiteral(key) && esutils.keyword.isIdentifierName(key.value)) {
+    key.type = "Identifier";
+    key.name = key.value;
+    delete key.value;
+
+    node.computed = false;
+  }
+};