add optional typeof symbol transformer

lib/6to5/transformation/templates/typeof.js

@@ -0,0 +1,3 @@
+(function (obj) {
+  return obj && obj.constructor === Symbol ? "symbol" : typeof obj;
+});

lib/6to5/transformation/transform.js

@@ -89,6 +89,7 @@ _.each({
   _moduleFormatter:          require("./transformers/_module-formatter"),
   useStrict:                 require("./transformers/use-strict"),
 
+  typeofSymbol:              require("./transformers/optional-typeof-symbol"),
   coreAliasing:              require("./transformers/optional-core-aliasing"),
   undefinedToVoid:           require("./transformers/optional-undefined-to-void"),
 

lib/6to5/transformation/transformers/optional-typeof-symbol.js

@@ -0,0 +1,7 @@
+var t = require("../../types");
+
+exports.UnaryExpression = function (node, parent, file, scope) {
+  if (node.operator === "typeof") {
+    return t.callExpression(file.addHelper("typeof"), [node.argument]);
+  }
+};

test/fixtures/transformation/optional-typeof-symbol/basic/actual.js

@@ -0,0 +1,2 @@
+var s = Symbol("s");
+assert.equal(typeof s, "symbol");

test/fixtures/transformation/optional-typeof-symbol/basic/exec.js

@@ -0,0 +1,2 @@
+var s = Symbol("s");
+assert.equal(typeof s, "symbol");

test/fixtures/transformation/optional-typeof-symbol/basic/expected.js

@@ -0,0 +1,8 @@
+"use strict";
+
+var _typeof = function (obj) {
+  return obj && obj.constructor === Symbol ? "symbol" : typeof obj;
+};
+
+var s = Symbol("s");
+assert.equal(_typeof(s), "symbol");