[vm] Resource Identifier fix loading units

The loading units were taken from the definitions instead of the
references.
This CL fixes that and adds test cases.

Add some doc comments and references to TODOs.

TEST=pkg/vm/test/transformations/resource_identifier_test.dart

Change-Id: I59a640344206555ae1b190521ee3e36d52199700
Cq-Include-Trybots: dart/try:pkg-linux-debug-try,pkg-linux-release-arm64-try,pkg-mac-release-try,pkg-mac-release-arm64-try,pkg-win-release-try,pkg-win-release-arm64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/364103
Commit-Queue: Daco Harkes <dacoharkes@google.com>
Reviewed-by: Moritz Sümmermann <mosum@google.com>

https://dart.googlesource.com/sdk/+/94890c86c7d2e6fa029ed735b366d2b05be4de86

DEPS

diff --git a/DEPS b/DEPS
index 032a2f4..b3d4e49 100644
--- a/DEPS
+++ b/DEPS
@@ -293,7 +293,7 @@
 
   'engine/src/flutter': Var('flutter_git') + '/mirrors/engine' + '@' + 'c410180e5bbaedd09b6d6fd34fafe79eb6456865',
 
-  'engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '54031e2752d76a95fa7fa069bf1b41a21260b5e9',
+  'engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '94890c86c7d2e6fa029ed735b366d2b05be4de86',
 
   'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '5f0628d335d630d1b9b890bf5460d4fa365e482e',
 

commits.json

diff --git a/commits.json b/commits.json
index b84e061..4e6d1ca 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "engine/src/flutter":"c410180e5bbaedd09b6d6fd34fafe79eb6456865",
-  "engine/src/flutter/third_party/dart":"54031e2752d76a95fa7fa069bf1b41a21260b5e9",
+  "engine/src/flutter/third_party/dart":"94890c86c7d2e6fa029ed735b366d2b05be4de86",
   "flutter":"5f0628d335d630d1b9b890bf5460d4fa365e482e"
 }
\ No newline at end of file