Add new Build header (#625)

Currently the `build` step gets jumbled up with the `npm install` step. Let's cleanly separate the two

bin/compile

@@ -257,6 +257,13 @@ build_dependencies() {
   mtime "modules.time.cache.$cache_status" "${start}"
 
   if [[ "$NEW_BUILD_SCRIPT_BEHAVIOR" = true ]]; then
+    if $YARN; then
+      header "Build"
+    else
+      # npm already prints a new line after the install
+      # without skipping the newline there's an awkward double line break
+      header_skip_newline "Build"
+    fi
     mcount "build-script.new-behavior"
     run_build_script "$BUILD_DIR"
   else

lib/output.sh

@@ -28,6 +28,10 @@ header() {
   echo "-----> $*" || true
 }
 
+header_skip_newline() {
+  echo "-----> $*" || true
+}
+
 error() {
   echo " !     $*" >&2 || true
   echo "" || true

test/fixtures/build-script-opt-in-yarn/README.md

+A fake README, to keep npm from polluting stderr.
\ No newline at end of file

test/fixtures/build-script-opt-in-yarn/package.json

+{
+  "name": "node-buildpack-test-app",
+  "version": "0.0.1",
+  "description": "node buildpack integration test app",
+  "repository" : {
+    "type" : "git",
+    "url" : "http://github.com/example/example.git"
+  },
+  "scripts" : {
+    "build" : "echo build hook message"
+  },
+  "heroku-run-build-script": true,
+  "license": "MIT"
+}

test/fixtures/build-script-opt-in-yarn/yarn.lock

+# THIS IS AN AUTOGENERATED FILE. DO NOT EDIT THIS FILE DIRECTLY.
+# yarn lockfile v1
+
+

test/run

@@ -72,6 +72,13 @@ testBuildScriptOptIn() {
   assertCapturedSuccess
 }
 
+testBuildScriptOptInYarn() {
+  compile "build-script-opt-in-yarn"
+  assertCaptured "Running build (yarn)"
+  assertCaptured "build hook message"
+  assertCapturedSuccess
+}
+
 testPreferEmptyHerokuPostbuildOverBuild() {
   compile "empty-heroku-postbuild"
   assertCaptured "Detected both \"build\" and \"heroku-postbuild\" scripts"