add corejs aliasing transformer and support for optional transformers

lib/6to5/transformation/templates/corejs-iterator.js

@@ -0,0 +1 @@
+CORE_ID.$for.getIterator(VALUE);

lib/6to5/transformation/transform.js

@@ -80,6 +80,8 @@ _.each({
 
   _declarations:             require("./transformers/_declarations"),
 
+  coreAliasing:              require("./transformers/optional-core-aliasing"),
+
   // wrap up
   _aliasFunctions:           require("./transformers/_alias-functions"),
   useStrict:                 require("./transformers/use-strict"),

lib/6to5/transformation/transformer.js

@@ -5,9 +5,11 @@ var t        = require("../types");
 var _        = require("lodash");
 
 function Transformer(key, transformer, opts) {
-  this.transformer = Transformer.normalise(transformer);
-  this.opts        = opts || {};
-  this.key         = key;
+  this.experimental = !!transformer.experimental;
+  this.transformer  = Transformer.normalise(transformer);
+  this.optional     = !!transformer.optional;
+  this.opts         = opts || {};
+  this.key          = key;
 }
 
 Transformer.normalise = function (transformer) {
@@ -16,8 +18,13 @@ Transformer.normalise = function (transformer) {
   }
 
   _.each(transformer, function (fns, type) {
+    // hidden property
     if (type[0] === "_") return;
+
     if (_.isFunction(fns)) fns = { enter: fns };
+
+    if (!_.isObject(fns)) return;
+
     transformer[type] = fns;
 
     var aliases = t.FLIPPED_ALIAS_KEYS[type];
@@ -41,13 +48,9 @@ Transformer.prototype.astRun = function (file, key) {
 };
 
 Transformer.prototype.transform = function (file) {
-  if (!this.canRun(file)) return;
-
   var transformer = this.transformer;
   var ast         = file.ast;
 
-  this.astRun(file, "enter");
-
   var build = function (exit) {
     return function (node, parent, scope) {
       var fns = transformer[node.type];
@@ -61,12 +64,14 @@ Transformer.prototype.transform = function (file) {
     };
   };
 
+  this.astRun(file, "before");
+
   traverse(ast, {
     enter: build(),
     exit:  build(true)
   });
 
-  this.astRun(file, "exit");
+  this.astRun(file, "after");
 };
 
 Transformer.prototype.canRun = function (file) {
@@ -80,5 +85,9 @@ Transformer.prototype.canRun = function (file) {
   var whitelist = opts.whitelist;
   if (whitelist.length && !_.contains(whitelist, key)) return false;
 
+  if (this.optional && !_.contains(opts.optional, key)) return false;
+
+  if (this.experimental && !opts.experimental) return false;
+
   return true;
 };

lib/6to5/types/builder-keys.json

@@ -11,6 +11,8 @@
   "FunctionExpression":      ["id", "params", "body", "generator"],
   "Identifier":              ["name"],
   "IfStatement":             ["test", "consequent", "alternate"],
+  "ImportDeclaration":       ["specifiers", "source"],
+  "ImportSpecifier":         ["id", "name"],
   "Literal":                 ["value"],
   "LogicalExpression":       ["operator", "left", "right"],
   "MemberExpression":        ["object", "property", "computed"],