DAS plugins: Move Assist code to analysis_server_plugin package

The PluginServer class needs access to AssistProcessor in order to
compute assists. That class needs access to a few other classes, which
then must also be moved:

* assist_core.dart - the Assist class
* assist_dart.dart - the DartAssistContext class
* assist_generators.dart - the registeredAssistGenerators variable
* assist_performance.dart - the AssistPerformance and
  GetAssistsPerformance classes
* assist_processor.dart - the AssistProcessor class with it's
  singular API, `compute()`
* performance.dart - the ProducerRequestPerformance class
* the `addCaretAtOffset` helper function, refactored into a
  `withCaretAt` extension method

This change is functionally a no-op.

Change-Id: Ic883d21e9cc8c3db1f6093f830f01ec6eb9a0976
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/416680
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Phil Quitslund <pquitslund@google.com>

https://dart.googlesource.com/sdk/+/331c700f33e2405b5ea05bdd274f392ee78be4fa
diff --git a/DEPS b/DEPS
index ff68273..6c99c2f 100644
--- a/DEPS
+++ b/DEPS
@@ -238,7 +238,7 @@
 ]
 
 deps = {
-  'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '0f2c6d73020cc7bfea36fdc85e3267c3ae4b798c',
+  'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '331c700f33e2405b5ea05bdd274f392ee78be4fa',
 
   'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + 'bf3826d2ce928dc8ed0d66c0aeb4c8c3a86705ab',
 
diff --git a/commits.json b/commits.json
index 39fbf0f..bda2016 100644
--- a/commits.json
+++ b/commits.json
@@ -1,4 +1,4 @@
 {
   "flutter":"bf3826d2ce928dc8ed0d66c0aeb4c8c3a86705ab",
-  "flutter/engine/src/flutter/third_party/dart":"0f2c6d73020cc7bfea36fdc85e3267c3ae4b798c"
+  "flutter/engine/src/flutter/third_party/dart":"331c700f33e2405b5ea05bdd274f392ee78be4fa"
 }
\ No newline at end of file