Adding API documentation downloads to the archive

Fixes #933 

Provides download links for the archived API docs. The links link to the zip file, which contains a directory of JSON files. This is currently the only format which we provide the API documentation.

The [dartdoc-viewer](https://github.com/dart-lang/dartdoc-viewer) README shows how these can be viewed and served.

This is staged at https://issue-933-api-downloads-dot-dart-lang.appspot.com/tools/download_archive/

@sethladd @kwalrath @Sfshaza let me know what you think.
Should this be part of the previous table? Maybe:


```
| Version           | OS    | Bits   | Downloads            |
| 1.6.0 (rev 39553) | Mac   | 32-bit | Dart SDK (SHA-256)   |
|                   |       |        | Dartium (SHA-256)    |
|                   |       |        | Dart Editor          |
| 1.6.0 (rev 39553) | Mac   | 64-bit | Dart SDK (SHA-256)   |
|                   |       |        | Dart Editor          |
| 1.6.0 (rev 39553) | Linux | 32-bit | Dart SDK (SHA-256)   |
|                   |       |        | Dartium (SHA-256)    |
|                   |       |        | Dart Editor          |
| 1.6.0 (rev 39553) | Linux | 64-bit | Dart SDK (SHA-256)   |
|                   |       |        | Dartium (SHA-256)    |
|                   |       |        | Dart Editor          |
| 1.6.0 (rev 39553) | Win   | 32-bit | Dart SDK (SHA-256)   |
|                   |       |        | Dartium (SHA-256)    |
|                   |       |        | Dart Editor          |
| 1.6.0 (rev 39553) | Win   | 64-bit | Dart SDK (SHA-256)   |
|                   |       |        | Dart Editor          |
| 1.6.0 (rev 39553) | —     | —      | API Reference (JSON) |
| 1.5.8 (rev 38663) | Mac   | 32-bit | Dart SDK             |
...
```