fixes #411 - add npm config platform env var in order to set which js…

Summary:
… platform will be built
Closes https://github.com/facebook/yoga/pull/537

Differential Revision: D5069356

Pulled By: emilsjolander

fbshipit-source-id: eaa67d17fe9d5ed9d0959781ed12a238cb3d95f2

javascript/package.json

@@ -11,6 +11,10 @@
     "main": "./sources/entry-node",
     "browser": "./sources/entry-browser",
 
+    "config": {
+        "platform": "all"
+    },
+
     "scripts": {
 
         "which": "which",
@@ -25,7 +29,7 @@
         "build:browser": "npm -- run copy-sources && npm -- run node-gyp configure build --asmjs=1",
         "build:standalone": "webpack",
         "build:all": "npm -- run build:node && npm -- run build:browser && npm -- run build:standalone",
-        "build": "npm -- run build:all",
+        "build": "cross-env \"npm --if-present -- run build:$npm_package_config_platform\"",
 
         "test:node": "TEST_ENTRY=node time mocha --expose-gc -r tests/tools.js tests/Facebook.Yoga/**/*.js",
         "test:browser": "TEST_ENTRY=browser time mocha --expose-gc -r tests/tools.js tests/Facebook.Yoga/**/*.js",
@@ -50,8 +54,8 @@
 
     "devDependencies": {
 
+        "cross-env": "^4.0.0",
         "mocha": "^3.2.0",
         "webpack": "^2.2.0-rc.2"
-
     }
 }