cache web assets between CI runs (#2089)

- skip `test/jetstream.js` for `node@0.12`

test/fetch.js

@@ -0,0 +1,31 @@
+var fs = require("fs");
+var path = require("path");
+
+try {
+    fs.mkdirSync("./tmp");
+} catch (e) {
+    if (e.code != "EEXIST") throw e;
+}
+
+function local(url) {
+    return path.join("./tmp", encodeURIComponent(url));
+}
+
+function read(url) {
+    return fs.createReadStream(local(url));
+}
+
+module.exports = function(url, callback) {
+    var result = read(url);
+    result.on("error", function(e) {
+        if (e.code != "ENOENT") return callback(e);
+        require(url.slice(0, url.indexOf(":"))).get(url, function(res) {
+            if (res.statusCode !== 200) return callback(res);
+            res.pipe(fs.createWriteStream(local(url)).on("close", function() {
+                callback(null, read(url));
+            }));
+        });
+    }).on("open", function() {
+        callback(null, result);
+    });
+};