Return Impl nodes from Impl nodes.

The goal is to remove operations that modify AST from API.

To do this we need resolution and utilities to use visitors that
provide Impl nodes, and sub-nodes that we access are also Impl nodes.

Change-Id: I4bd9aee8a4c6668e4242df9aa41cd572a1c49bef
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/189440
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

https://dart.googlesource.com/sdk/+/e26bd4471ecc6f5fffdf8883bcf4b946d5baed1d
diff --git a/commits.json b/commits.json
index 897e12d..4167a98 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "external/github.com/flutter/engine":"09b2468785a3e07132cac1fba99b1beafdf9dcec",
   "external/github.com/flutter/flutter":"a0ba6464089f906fefbf610732b2c9a7b66a1260",
-  "sdk":"b3de362dc5b2690f10d79de8a2117f699a92058c"
+  "sdk":"e26bd4471ecc6f5fffdf8883bcf4b946d5baed1d"
 }
\ No newline at end of file