Refactor summary IDL generator.

I need to make changes to 'collectApiSignature' and 'flushInformative'
to support variants, and we have lots of code around already.

R=brianwilkerson@google.com, paulberry@google.com

Change-Id: I9ba3b61a22b9e9e2c547e97d5c5adaa07443b24a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/103552
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

https://dart.googlesource.com/sdk/+/8208e872f829e71b7e2eb2953f08515e30b20e0a
diff --git a/commits.json b/commits.json
index b64030c..377267d 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "external/github.com/flutter/engine":"ee6a9c40492e54a0ae160293900d9e3b42b725d7",
   "external/github.com/flutter/flutter":"1bd85dd6c207d69c148b92ec8f68244c00dec503",
-  "sdk":"22c086fbe56102e304197c1d52fbd92d0efcb1f0"
+  "sdk":"8208e872f829e71b7e2eb2953f08515e30b20e0a"
 }
\ No newline at end of file