add binding for keys method

tests/all/js_globals/ArrayIterator.rs

@@ -0,0 +1,34 @@
+#![allow(non_snake_case)]
+
+use project;
+
+#[test]
+fn keys() {
+    project()
+        .file("src/lib.rs", r#"
+            #![feature(proc_macro, wasm_custom_section)]
+
+            extern crate wasm_bindgen;
+            use wasm_bindgen::prelude::*;
+            use wasm_bindgen::js;
+
+            #[wasm_bindgen]
+            pub fn get_keys(this: &js::Array) -> js::ArrayIterator {
+                this.keys()
+            }
+
+        "#)
+        .file("test.ts", r#"
+            import * as assert from "assert";
+            import * as wasm from "./out";
+
+            export function test() {
+                let characters = [8, 5, 4, 3, 1, 2]
+                let iterator = characters.keys();
+                let wasmIterator = wasm.get_keys(characters);
+
+                assert.equal(iterator.toString(), wasmIterator.toString());
+            }
+        "#)
+        .test()
+}