feat(js) Implement the Date.now binding.

src/js.rs

@@ -431,6 +431,13 @@ extern {
     #[wasm_bindgen(constructor)]
     pub fn new() -> Date;
 
+    /// The `Date.now()` method returns the number of milliseconds
+    /// elapsed since January 1, 1970 00:00:00 UTC.
+    ///
+    /// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/now
+    #[wasm_bindgen(static_method_of = Date)]
+    pub fn now() -> Number;
+
     /// The toDateString() method returns the date portion of a Date object
     /// in human readable form in American English.
     /// 

tests/all/js_globals/Date.rs

@@ -28,6 +28,32 @@ fn new() {
         .test()
 }
 
+#[test]
+fn now() {
+    project()
+        .file("src/lib.rs", r#"
+            #![feature(proc_macro, wasm_custom_section)]
+
+            extern crate wasm_bindgen;
+            use wasm_bindgen::prelude::*;
+            use wasm_bindgen::js::{Date, Number};
+
+            #[wasm_bindgen]
+            pub fn now() -> Number {
+                Date::now()
+            }
+        "#)
+        .file("test.ts", r#"
+            import * as assert from "assert";
+            import * as wasm from "./out";
+
+            export function test() {
+                assert.equal(typeof wasm.now(), "number");
+            }
+        "#)
+        .test()
+}
+
 #[test]
 fn to_date_string() {
     project()