Use an extra layer of indirection on handle

This fixes problems with very nested promises without recreating the
memory leak

Author: Forbes Lindesay

lib/core.js

@@ -27,8 +27,10 @@ function Promise(fn) {
     })
   }
   this.then[handleSymbol] = handle;
-
   function handle(deferred) {
+    return internalHandle(deferred);
+  }
+  function internalHandle(deferred) {
     if (state === null) {
       deferreds.push(deferred)
       return
@@ -61,10 +63,9 @@ function Promise(fn) {
             // to prevent a memory leak, we adopt the value of the other promise
             // allowing this promise to be garbage collected as soon as nobody
             // has a reference to it
-            handle = (self[handleSymbol] = then[handleSymbol]);
-            self.then = then;
+            internalHandle = then[handleSymbol];
             deferreds.forEach(function (deferred) {
-              handle(deferred);
+              internalHandle(deferred);
             });
           } else {
             doResolve(then.bind(newValue), resolve, reject)