add exponentiation operator

lib/6to5/transformation/transform.js

@@ -39,6 +39,7 @@ _.each({
   computedPropertyNames:     require("./transformers/es6-computed-property-names"),
 
   objectSpread:              require("./transformers/es7-object-spread"),
+  exponentiationOperator:    require("./transformers/es7-exponentiation-operator"),
   spread:                    require("./transformers/es6-spread"),
   templateLiterals:          require("./transformers/es6-template-literals"),
   propertyMethodAssignment:  require("./transformers/es5-property-method-assignment"),

lib/6to5/transformation/transformers/es7-exponentiation-operator.js

@@ -0,0 +1,16 @@
+// https://github.com/rwaldron/exponentiation-operator
+
+var t = require("../../types");
+var pow = t.memberExpression(t.identifier("Math"), t.identifier("pow"));
+
+exports.AssignmentExpression = function (node) {
+  if (node.operator !== "**=") return;
+  node.operator = "=";
+  node.right = t.callExpression(pow, [node.left, node.right]);
+};
+
+exports.BinaryExpression = function (node) {
+  if (node.operator !== "**") return;
+
+  return t.callExpression(pow, [node.left, node.right]);
+};