flow-lib: JS interfaces for Flow APIs

Summary:
This is a first-pass on an idea for an npm package, called `flow-lib`, that allows JS tooling consumers to interface easily with Flow APIs. Pending more experimentation, my hope is that this can become a stable way for JS tooling to integrate with Flow via a simple, well-versioned npm dependency.

To start out I've added JS APIs for `flow check-contents`, `flow ast`, and Flow type definitions for the AST. I also intend to add `type-at-pos`-like APIs as well (all of the tooling needs I can imagine right now need this more than anything else) -- but before that I need to find a good way to represent types that isn't coupled too tightly with Flow implementation details. I'm still working on this and will update when I have something more here.

**NOTE: This is only an RFC while I look into more concrete use cases. I am putting this out for discussion, but I don't necessarily expect to merge it until we see it prove useful or promising for some *specific* tooling needs**
Closes facebook#1360

Reviewed By: samwgoldman

Differential Revision: D3676384

Pulled By: jeffmo

fbshipit-source-id: 96925ebcf12d0509fe49eaa1e17d7958edc20ada

Author: jeffmo

npm-flow-lib/.babelrc

@@ -0,0 +1,8 @@
+{
+  "plugins": [
+    "transform-flow-strip-types",
+    "transform-regenerator",
+    "syntax-async-functions",
+   ],
+  "presets": ["es2015"]
+}

npm-flow-lib/.flowconfig

@@ -0,0 +1,3 @@
+node_modules
+*.swp
+dist

npm-flow-lib/.gitignore

@@ -0,0 +1 @@
+src/

npm-flow-lib/.npmignore

@@ -0,0 +1,22 @@
+{
+  "name": "flow-lib",
+  "version": "0.0.0-pre",
+  "main": "dist/index.js",
+  "dependencies": {
+    "babel-polyfill": "^6.13.0"
+  },
+  "devDependencies": {
+    "babel-cli": "^6.11.4",
+    "babel-plugin-syntax-async-functions": "^6.13.0",
+    "babel-plugin-transform-flow-strip-types": "^6.8.0",
+    "flow-bin": "^0.30.0"
+  },
+  "scripts": {
+    "build": "mkdir -p dist; babel ./src --out-dir=./dist",
+    "clean": "rm -rf dist",
+    "flow": "flow; test $? -eq 0 -o $? -eq 2",
+    "prepublish": "mkdir -p; npm run test",
+    "test": "node dist/test_ast_types",
+    "watch": "mkdir -p dist; babel --watch=./src --out-dir=./dist"
+  }
+}