further work splitting up transformers into their own "category" in order of what they need and what they actually do

src/babel/transformation/transformers/es3/member-expression-literals.js

@@ -1,7 +1,7 @@
 import * as t from "../../../types";
 
 export var metadata = {
-  category: "builtin-cleanup"
+  category: "builtin-trailing"
 };
 
 export var MemberExpression = {

src/babel/transformation/transformers/es3/property-literals.js

@@ -1,7 +1,7 @@
 import * as t from "../../../types";
 
 export var metadata = {
-  category: "builtin-cleanup"
+  category: "builtin-trailing"
 };
 
 export var Property = {

src/babel/transformation/transformers/es5/properties.mutators.js

@@ -1,26 +1,24 @@
 import * as defineMap from "../../helpers/define-map";
 import * as t from "../../../types";
 
-export var ObjectExpression = {
-  exit(node, parent, scope, file) {
-    var mutatorMap = {};
-    var hasAny = false;
+export function ObjectExpression(node, parent, scope, file) {
+  var mutatorMap = {};
+  var hasAny = false;
 
-    node.properties = node.properties.filter(function (prop) {
-      if (prop.kind === "get" || prop.kind === "set") {
-        hasAny = true;
-        defineMap.push(mutatorMap, prop, prop.kind, file);
-        return false;
-      } else {
-        return true;
-      }
-    });
+  node.properties = node.properties.filter(function (prop) {
+    if (prop.kind === "get" || prop.kind === "set") {
+      hasAny = true;
+      defineMap.push(mutatorMap, prop, prop.kind, file);
+      return false;
+    } else {
+      return true;
+    }
+  });
 
-    if (!hasAny) return;
+  if (!hasAny) return;
 
-    return t.callExpression(
-      t.memberExpression(t.identifier("Object"), t.identifier("defineProperties")),
-      [node, defineMap.toDefineObject(mutatorMap)]
-    );
-  }
-};
+  return t.callExpression(
+    t.memberExpression(t.identifier("Object"), t.identifier("defineProperties")),
+    [node, defineMap.toDefineObject(mutatorMap)]
+  );
+}

src/babel/transformation/transformers/es6/destructuring.js

@@ -1,6 +1,10 @@
 import * as messages from "../../../messages";
 import * as t from "../../../types";
 
+export var metadata = {
+  category: "builtin-advanced"
+};
+
 export function ForOfStatement(node, parent, scope, file) {
   var left = node.left;
 

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

@@ -67,8 +67,8 @@ export var ObjectExpression = {
   exit(node, parent, scope, file) {
     var hasComputed = false;
 
-    for (var i = 0; i < node.properties.length; i++) {
-      hasComputed = t.isProperty(node.properties[i], { computed: true, kind: "init" });
+    for (var prop of (node.properties: Array)) {
+      hasComputed = t.isProperty(prop, { computed: true, kind: "init" });
       if (hasComputed) break;
     }
 

src/babel/transformation/transformers/es6/tail-call.js

@@ -6,6 +6,10 @@ import * as util from  "../../../util";
 import map from "lodash/collection/map";
 import * as t from "../../../types";
 
+export var metadata = {
+  category: "builtin-trailing"
+};
+
 exports.Function = function (node, parent, scope, file) {
   if (node.generator || node.async) return;
   var tailCall = new TailCallTransformer(this, scope, file);

src/babel/transformation/transformers/index.js

@@ -1,20 +1,24 @@
 export default {
-  // builtin-basic
+  //- builtin-setup
+  _validation:                             require("./internal/validation"),
   "utility.removeDebugger":                require("./utility/remove-debugger"),
   "utility.removeConsole":                 require("./utility/remove-console"),
   "utility.inlineEnvironmentVariables":    require("./utility/inline-environment-variables"),
   "utility.inlineExpressions":             require("./utility/inline-expressions"),
   "minification.deadCodeElimination":      require("./minification/dead-code-elimination"),
   _modules:                                require("./internal/modules"),
+  "spec.functionName":                     require("./spec/function-name"),
+
+  //- builtin-basic
+  // this is where the bulk of the ES6 transformations take place, none of them require traversal state
+  // so they can all be concatenated together for performance
   "es7.classProperties":                   require("./es7/class-properties"),
   "es7.trailingFunctionCommas":            require("./es7/trailing-function-commas"),
   "es7.asyncFunctions":                    require("./es7/async-functions"),
   "es7.decorators":                        require("./es7/decorators"),
   strict:                                  require("./other/strict"),
-  _validation:                             require("./internal/validation"),
   "validation.undeclaredVariableCheck":    require("./validation/undeclared-variable-check"),
   "validation.react":                      require("./validation/react"),
-  "spec.functionName":                     require("./spec/function-name"),
   "es6.arrowFunctions":                    require("./es6/arrow-functions"),
   "spec.blockScopedFunctions":             require("./spec/block-scoped-functions"),
   "optimisation.react.constantElements":   require("./optimisation/react.constant-elements"),
@@ -41,30 +45,37 @@ export default {
   "es6.parameters.rest":                   require("./es6/parameters.rest"),
   "es6.spread":                            require("./es6/spread"),
   "es6.parameters.default":                require("./es6/parameters.default"),
-  "es6.destructuring":                     require("./es6/destructuring"),
-  "es6.tailCall":                          require("./es6/tail-call"),
   "es7.exportExtensions":                  require("./es7/export-extensions"),
   "spec.protoToAssign":                    require("./spec/proto-to-assign"),
-  _shadowFunctions:                        require("./internal/shadow-functions"),
   "es7.doExpressions":                     require("./es7/do-expressions"),
   "es6.spec.symbols":                      require("./es6/spec.symbols"),
-  ludicrous:                               require("./other/ludicrous"),
   "spec.undefinedToVoid":                  require("./spec/undefined-to-void"),
   jscript:                                 require("./other/jscript"),
   flow:                                    require("./other/flow"),
   _hoistDirectives:                        require("./internal/hoist-directives"),
 
-  // builtin-modules
-  "es6.modules":                           require("./es6/modules"),
-  regenerator:                             require("./other/regenerator"),
-  runtime:                                 require("./other/runtime"),
-  _moduleFormatter:                        require("./internal/module-formatter"),
-
-  // builtin-advanced
+  //- builtin-advanced
+  "es6.destructuring":                     require("./es6/destructuring"),
   "es6.blockScoping":                      require("./es6/block-scoping"),
   "es6.spec.blockScoping":                 require("./es6/spec.block-scoping"),
 
-  // builtin-cleanup
+  // es6 syntax transformation is **forbidden** past this point since regenerator will chuck a massive
+  // hissy fit
+
+  //- regenerator
+  regenerator:                             require("./other/regenerator"),
+
+  //- builtin-modules
+  runtime:                                 require("./other/runtime"),
+  "es6.modules":                           require("./es6/modules"),
+  _moduleFormatter:                        require("./internal/module-formatter"),
+
+  //- builtin-trailing
+  // these clean up the output and do finishing up transformations, it's important to note that by this
+  // stage you can't import any new modules or insert new ES6 as all those transformers have already
+  // been ran
+  "es6.tailCall":                          require("./es6/tail-call"),
+  _shadowFunctions:                        require("./internal/shadow-functions"),
   "es3.propertyLiterals":                  require("./es3/property-literals"),
   "es3.memberExpressionLiterals":          require("./es3/member-expression-literals"),
   "minification.memberExpressionLiterals": require("./minification/member-expression-literals"),

src/babel/transformation/transformers/internal/block-hoist.js

@@ -1,7 +1,7 @@
 import sortBy from "lodash/collection/sortBy";
 
 export var metadata = {
-  category: "builtin-cleanup"
+  category: "builtin-trailing"
 };
 
 // Priority:

src/babel/transformation/transformers/internal/modules.js

@@ -6,6 +6,10 @@
 
 import * as t from "../../../types";
 
+export var metadata = {
+  category: "builtin-setup"
+};
+
 export function ImportDeclaration(node, parent, scope, file) {
   if (node.source) {
     node.source.value = file.resolveModuleSource(node.source.value);

src/babel/transformation/transformers/internal/shadow-functions.js

@@ -86,7 +86,7 @@ function aliasFunction(getBody, path, scope) {
 // crawling the entire function tree
 
 export var metadata = {
-  category: "builtin-cleanup"
+  category: "builtin-trailing"
 };
 
 export var Program = {

src/babel/transformation/transformers/internal/validation.js

@@ -1,6 +1,10 @@
 import * as messages from "../../../messages";
 import * as t from "../../../types";
 
+export var metadata = {
+  category: "builtin-setup"
+};
+
 export function ForOfStatement(node, parent, scope, file) {
   var left = node.left;
   if (t.isVariableDeclaration(left)) {

src/babel/transformation/transformers/minification/dead-code-elimination.js

@@ -18,7 +18,8 @@ function toStatements(node) {
 }
 
 export var metadata = {
-  optional: true
+  optional: true,
+  category: "builtin-setup"
 };
 
 export function Identifier(node, parent, scope) {

src/babel/transformation/transformers/minification/member-expression-literals.js

@@ -2,7 +2,7 @@ import * as t from "../../../types";
 
 export var metadata = {
   optional: true,
-  category: "builtin-cleanup"
+  category: "builtin-trailing"
 };
 
 export var MemberExpression = {

src/babel/transformation/transformers/minification/property-literals.js

@@ -2,7 +2,7 @@ import * as t from "../../../types";
 
 export var metadata = {
   optional: true,
-  category: "builtin-cleanup"
+  category: "builtin-trailing"
 };
 
 export var Property = {

src/babel/transformation/transformers/other/regenerator.js

@@ -2,11 +2,9 @@ import regenerator from "regenerator";
 import * as t from "../../../types";
 
 export var metadata = {
-  category: "builtin-modules"
+  category: "regenerator"
 };
 
-export var Program = {
-  exit(ast) {
-    regenerator.transform(ast);
-  }
-};
+export function Program(ast) {
+  regenerator.transform(ast);
+}

src/babel/transformation/transformers/other/runtime/index.js

@@ -11,10 +11,10 @@ const RUNTIME_MODULE_NAME = "babel-runtime";
 
 export var metadata = {
   optional: true,
-  category: "builtin-modules"
+  category: "builtin-post-modules"
 };
 
-export function Program(node, parent, scope, file) {
+export function pre(file) {
   file.set("helperGenerator", function (name) {
     return file.addImport(`${RUNTIME_MODULE_NAME}/helpers/${name}`, name, "absoluteDefault");
   });

src/babel/transformation/transformers/spec/function-name.js

@@ -1,3 +1,7 @@
+export var metadata = {
+  category: "builtin-setup"
+};
+
 export {
   bare as FunctionExpression,
   bare as ArrowFunctionExpression

src/babel/transformation/transformers/utility/inline-environment-variables.js

@@ -1,7 +1,8 @@
 import * as t from "../../../types";
 
 export var metadata = {
-  optional: true
+  optional: true,
+  category: "builtin-setup"
 };
 
 var match = t.buildMatchMemberExpression("process.env");

src/babel/transformation/transformers/utility/inline-expressions.js

@@ -1,7 +1,8 @@
 import * as t from "../../../types";
 
 export var metadata = {
-  optional: true
+  optional: true,
+  category: "builtin-setup"
 };
 
 export function Expression(node, parent, scope) {

src/babel/transformation/transformers/utility/remove-console.js

@@ -1,7 +1,8 @@
 import * as t from "../../../types";
 
 export var metadata = {
-  optional: true
+  optional: true,
+  category: "builtin-setup"
 };
 
 export function CallExpression(node, parent) {

src/babel/transformation/transformers/utility/remove-debugger.js

@@ -1,7 +1,8 @@
 import * as t from "../../../types";
 
 export var metadata = {
-  optional: true
+  optional: true,
+  category: "builtin-setup"
 };
 
 export function ExpressionStatement(node) {