test and implementation to skip custom npm versions with iojs

bin/compile

@@ -76,11 +76,12 @@ install_bins() {
   if [ -n "$iojs_engine" ]; then
     warn_node_engine "$iojs_engine"
     install_iojs "$iojs_engine" "$BUILD_DIR/.heroku/node"
+    echo "Using bundled npm version for iojs compatibility: `npm --version`"
   else
     warn_node_engine "$node_engine"
     install_nodejs "$node_engine" "$BUILD_DIR/.heroku/node"
+    install_npm "$npm_engine" "$BUILD_DIR/.heroku/node"
   fi
-  install_npm "$npm_engine" "$BUILD_DIR/.heroku/node"
   warn_old_npm
 }
 

test/fixtures/iojs/package.json

@@ -10,7 +10,8 @@
     "hashish": "*"
   },
   "engines": {
-    "iojs": "1.0.x"
+    "iojs": "1.0.x",
+    "npm": "2.11.x"
   },
   "scripts": {
     "postinstall": "echo \"detected node version:\" `node --version`"

test/run

@@ -58,6 +58,7 @@ testIoJs() {
   compile "iojs"
   assertCaptured "engines.iojs (package.json):  1.0."
   assertCaptured "Downloading and installing iojs 1.0."
+  assertNotCaptured "Downloading and installing npm"
   assertCapturedSuccess
 }