Keep more specific types for AnalysisContextCollectionImpl to avoid downcasts.

It seems to me that implicitly expecting DriverBasedAnalysisContext
is equivalent to explicitly stating this with types.

Change-Id: I16ec14e73030d9b34242a2f4ad582e93fc44afa7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/189382
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

https://dart.googlesource.com/sdk/+/53c8eda9672fe1555821bfa51e31f3c57683da1d
diff --git a/commits.json b/commits.json
index 9a72a2c..42a44c4 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "external/github.com/flutter/engine":"9662a38a79732fc9090bb43a47003d8e063ee517",
   "external/github.com/flutter/flutter":"a0ba6464089f906fefbf610732b2c9a7b66a1260",
-  "sdk":"c8d6f85d0f70fb870e7396fbc224817973766552"
+  "sdk":"53c8eda9672fe1555821bfa51e31f3c57683da1d"
 }
\ No newline at end of file