Add flow types definitions

.gitignore

@@ -1 +1,2 @@
 /node_modules
+/dist/ResizeObserver.js.flow

package.json

@@ -7,7 +7,7 @@
     "jsnext:main": "dist/ResizeObserver.es.js",
     "module": "dist/ResizeObserver.es.js",
     "scripts": {
-        "build": "rollup -c",
+        "build": "rollup -c && cpy src/index.js.flow dist --rename=ResizeObserver.js.flow",
         "test": "npm run test:lint && npm run test:spec",
         "test:ci": "npm run test:lint && npm run test:spec:sauce && npm run test:spec:node",
         "test:lint": "node ./node_modules/eslint/bin/eslint.js \"**/*.js\" --ignore-pattern \"/dist/\"",
@@ -48,6 +48,7 @@
         "babel-plugin-transform-regenerator": "^6.22.0",
         "babel-preset-latest": "^6.24.0",
         "babel-preset-stage-2": "^6.22.0",
+        "cpy-cli": "^1.0.1",
         "eslint": "^3.19.0",
         "jasmine": "^2.5.3",
         "jasmine-core": "^2.5.2",

src/index.js.flow

@@ -0,0 +1,32 @@
+// @flow
+
+class DOMRectReadOnly {
+  +x: number;
+  +y: number;
+  +width: number;
+  +height: number;
+  +top: number;
+  +right: number;
+  +bottom: number;
+  +left: number;
+}
+
+class ResizeObserverEntry {
+  +target: Element;
+  +contentRect: DOMRectReadOnly;
+}
+
+type Entries = $ReadOnlyArray<ResizeObserverEntry>;
+
+type ResizeObserverCallback = {
+  (entries: Entries, observer: ResizeObserver): void
+};
+
+declare class ResizeObserver {
+  constructor(ResizeObserverCallback): void;
+  observe(target: Element): void;
+  unobserve(target: Element): void;
+  disconnect(): void;
+}
+
+export default typeof ResizeObserver