absolute FileAttachment.url (#926)

* absolute FileAttachment.url

* update docs

Author: mbostock

docs/javascript/files.md

@@ -97,7 +97,7 @@ None of the files in `frames` above are loaded until a [content method](#support
 
 The contents often dictate the appropriate method — for example, an Apache Arrow file is almost always read with `file.arrow`. When multiple methods are valid, choose based on your needs. For example, you can load a CSV file using `file.text` to implement parsing yourself instead of using D3.
 
-In addition to the above, you can get the resolved relative path to the file using `file.url`. This returns a [promise](./promises) to a string:
+In addition to the above, you can get the resolved absolute URL of the file using `file.url`. This returns a [promise](./promises) to a string:
 
 ```js echo
 FileAttachment("volcano.json").url()

src/client/stdlib/fileAttachment.js

@@ -12,7 +12,7 @@ export function FileAttachment(name, base = location.href) {
   const file = files.get(url);
   if (!file) throw new Error(`File not found: ${name}`);
   const {path, mimeType} = file;
-  return new FileAttachmentImpl(path, name.split("/").pop(), mimeType);
+  return new FileAttachmentImpl(String(new URL(path, base)), name.split("/").pop(), mimeType);
 }
 
 async function remote_fetch(file) {