[cfe] Clean up backend imports

This updates backend imports to use reexports from api_prototype
or api_unstable.

TEST=existing

Change-Id: I8d9d1c76ef72c709c578acac5497064710ee5579
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/373462
Reviewed-by: Jens Johansen <jensj@google.com>
Reviewed-by: Srujan Gaddam <srujzs@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Ömer Ağacan <omersa@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>

https://dart.googlesource.com/sdk/+/c17e9c116e245d72bcf0928309293c51550ba53b

DEPS

diff --git a/DEPS b/DEPS
index 43c0d84..f2dcf9b 100644
--- a/DEPS
+++ b/DEPS
@@ -282,7 +282,7 @@
 
   'engine/src/flutter': Var('flutter_git') + '/mirrors/engine' + '@' + '8fcb3fe005deee9403d986688c84412fffe3d24b',
 
-  'engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '265197a85d4587078cc3a8ee0c16ea8854124028',
+  'engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + 'c17e9c116e245d72bcf0928309293c51550ba53b',
 
   'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '3cffdf6c438ad87c13219d3aea0cbeacac82acb8',
 

commits.json

diff --git a/commits.json b/commits.json
index 57a3449..5e1cdfc 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "engine/src/flutter":"8fcb3fe005deee9403d986688c84412fffe3d24b",
-  "engine/src/flutter/third_party/dart":"265197a85d4587078cc3a8ee0c16ea8854124028",
+  "engine/src/flutter/third_party/dart":"c17e9c116e245d72bcf0928309293c51550ba53b",
   "flutter":"3cffdf6c438ad87c13219d3aea0cbeacac82acb8"
 }
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