Add support for decoding URL-safe encoded base64.

Add support for decoding URL-safe encoded base64.

Also support "%3D" instead of "=" as padding, but no other percent escapes.

This is only for decoding, encoding is still as strict as possible.

R=nweiz@google.com

Committed: https://github.com/dart-lang/sdk/commit/289a22182e629e7e4f9085e3807ca0256dad5769

[Lasse Reichstein Nielsen, 2015-10-22 09:34:17]

lrn@google.com changed reviewers:
+ floitsch@google.com, nweiz@google.com

[nweiz, 2015-10-22 20:14:30]

lgtm

https://codereview.chromium.org/1419983003/diff/1/sdk/lib/convert/base64.dart
File sdk/lib/convert/base64.dart (right):

https://codereview.chromium.org/1419983003/diff/1/sdk/lib/convert/base64.dart...
sdk/lib/convert/base64.dart:574: * it needs when input is valid, and at least
enough bytes reach the error
"bytes reach" -> "bytes to reach"

https://codereview.chromium.org/1419983003/diff/1/sdk/lib/convert/base64.dart...
sdk/lib/convert/base64.dart:618: * That means zero or one padding character
(depending on [_state])
Aren't two padding characters also valid?

[Lasse Reichstein Nielsen, 2015-10-23 11:03:35]

https://codereview.chromium.org/1419983003/diff/1/sdk/lib/convert/base64.dart
File sdk/lib/convert/base64.dart (right):

https://codereview.chromium.org/1419983003/diff/1/sdk/lib/convert/base64.dart...
sdk/lib/convert/base64.dart:574: * it needs when input is valid, and at least
enough bytes reach the error
On 2015/10/22 20:14:30, nweiz wrote:
> "bytes reach" -> "bytes to reach"

Done.

https://codereview.chromium.org/1419983003/diff/1/sdk/lib/convert/base64.dart...
sdk/lib/convert/base64.dart:618: * That means zero or one padding character
(depending on [_state])
Yes, but we only get here after seeing the first one.
This text should be updated, because now we also get here after seeing the first
"%", so there is 1 2/3 padding left.
I've rewritten it completely.

[Lasse Reichstein Nielsen, 2015-10-26 08:41:35]

Committed patchset #3 (id:40001) manually as
289a22182e629e7e4f9085e3807ca0256dad5769 (presubmit successful).