Make yarn available to application if defined in engines (#404)

* Make yarn available to application if defined in engines

Fixes #402 

bin/compile

@@ -103,8 +103,15 @@ install_bins() {
     install_npm "$npm_engine" "$BUILD_DIR/.heroku/node"
     mcount "version.node.$(node --version)"
   fi
-  if $YARN; then
+
+  # Download yarn if there is a yarn.lock file or if the user
+  # has specified a version of yarn under "engines". We'll still
+  # only install using yarn if there is a yarn.lock file
+  if $YARN || [ -n "$yarn_engine" ]; then
     install_yarn "$BUILD_DIR/.heroku/yarn" "$yarn_engine"
+  fi
+
+  if $YARN; then
     mcount "version.yarn.$(yarn --version)"
   else
     mcount "version.npm.$(npm --version)"

test/fixtures/yarn-only-engine/package.json

+{
+  "name": "yarn",
+  "version": "1.0.0",
+  "main": "index.js",
+  "license": "MIT",
+  "engines": {
+    "yarn": "0.24.5"
+  },
+  "dependencies": {
+    "lodash": "^4.16.4"
+  }
+}

test/run

@@ -88,6 +88,15 @@ testYarnEngine() {
   assertCapturedSuccess
 }
 
+# If they specify a version of yarn inside package.json but
+# don't have a yarn.lock file download and make yarn available
+# though we will only install using yarn if a yarn.lock exists
+testYarnOnlyEngine() {
+  compile "yarn-only-engine"
+  assertCaptured "installing yarn (0.24.5)"
+  assertCapturedSuccess
+}
+
 testWarnUnmetDepNpm() {
   compile "unmet-dep"
   assertCaptured "fail npm install"