[dart2js ddc] make paths of dart2js and ddc platform files agnostic of nnbd.

Our platform .dill files show machine agnostic paths of the form:

  org-dartlang-sdk:///sdk/lib/foo/

and with the unfork:

  org-dartlang-sdk:///sdk_nnbd/lib/foo/

This change makes the path agnostic of the sdk_nnbd sources as well:

  org-dartlang-sdk:///lib/foo/

So that source-mapping tools can find the sources if needed (our sdk_nnbd vs sdk
is not visible on published sdks).

Fixes https://github.com/dart-lang/sdk/issues/41949

Change-Id: Iceea8a917fb9a6e5f305d22ba6ee0927354d4d8f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/148881
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Commit-Queue: Sigmund Cherem <sigmund@google.com>

https://dart.googlesource.com/sdk/+/521acb17e477ca02d61fa89800cfdda949e362eb
diff --git a/commits.json b/commits.json
index 47171ad..3d3b44d 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "external/github.com/flutter/engine":"368fe27d23dc34b93ae38414c5d6a508851829c6",
   "external/github.com/flutter/flutter":"5663e02645a6e69fedfe8553cbe7940cbb592be9",
-  "sdk":"1fba93f7c72eefae3701a63381aab521ffd79729"
+  "sdk":"521acb17e477ca02d61fa89800cfdda949e362eb"
 }
\ No newline at end of file