Add convienient broccoli debug tooling.

Allows easy debuging of input/output of babel trees via:

```js
BROCCOLI_DEBUG=ember-cli-babel:* ember b
```

index.js

@@ -5,6 +5,8 @@ const VersionChecker = require('ember-cli-version-checker');
 const clone = require('clone');
 const path = require('path');
 
+let count = 0;
+
 function addBaseDir(Plugin) {
   let type = typeof Plugin;
 
@@ -34,8 +36,22 @@ module.exports = {
     return this._getBabelOptions(config);
   },
 
-  transpileTree(tree, config) {
-    return require('broccoli-babel-transpiler')(tree, this.buildBabelOptions(config));
+  _debugTree() {
+    if (!this._cachedDebugTree) {
+      this._cachedDebugTree = require('broccoli-debug').buildDebugCallback(`ember-cli-babel:${this._parentName()}`);
+    }
+
+    return this._cachedDebugTree.apply(null, arguments);
+  },
+
+  transpileTree(inputTree, config) {
+    let description = `000${++count}`.slice(-3);
+    let postDebugTree = this._debugTree(inputTree, `${description}:input`);
+
+    let BabelTranspiler = require('broccoli-babel-transpiler');
+    let output = new BabelTranspiler(postDebugTree, this.buildBabelOptions(config));
+
+    return this._debugTree(output, `${description}:output`);
   },
 
   setupPreprocessorRegistry: function(type, registry) {

package.json

@@ -41,6 +41,7 @@
     "babel-polyfill": "^6.16.0",
     "babel-preset-env": "^1.5.1",
     "broccoli-babel-transpiler": "^6.0.0",
+    "broccoli-debug": "^0.6.2",
     "broccoli-funnel": "^1.0.0",
     "broccoli-source": "^1.1.0",
     "clone": "^2.0.0",

yarn.lock

@@ -951,6 +951,18 @@ broccoli-debug@^0.6.1:
     symlink-or-copy "^1.1.8"
     tree-sync "^1.2.2"
 
+broccoli-debug@^0.6.2:
+  version "0.6.2"
+  resolved "https://registry.yarnpkg.com/broccoli-debug/-/broccoli-debug-0.6.2.tgz#4c6e89459fc3de7d5d4fc7b77e57f46019f44db1"
+  dependencies:
+    broccoli-plugin "^1.2.1"
+    fs-tree-diff "^0.5.2"
+    heimdalljs "^0.2.1"
+    heimdalljs-logger "^0.1.7"
+    minimatch "^3.0.3"
+    symlink-or-copy "^1.1.8"
+    tree-sync "^1.2.2"
+
 broccoli-funnel-reducer@^1.0.0:
   version "1.0.0"
   resolved "https://registry.npmjs.org/broccoli-funnel-reducer/-/broccoli-funnel-reducer-1.0.0.tgz#11365b2a785aec9b17972a36df87eef24c5cc0ea"