[analyzer] Remove test class extending DiagnosticReporter.

Previous to this commit, `lint_rule_test.dart` contained a class,
`CollectingReporter`, which extended `DiagnosticReporter` and modified
its behavior so that instead of reporting the error to its listener,
it simply recorded the error code, so that tests could verify that an
error with the appropriate error code was reported.

It's not really necessary to have a whole class for this; the same
effect can be more easily accomplished by simply looking at the error
recorded by the `GatheringDiagnosticListener`, and extracting the
error code from it.

Eliminating the `CollectingReporter` class should make it easier to
refactor and add functionality to the `DiagnosticReporter` class,
which I intend to do as part of re-working the analyzer's error
reporting mechanism to be more type safe.

Change-Id: Ife8172da39bc9157a917c9b2d36cb3ee53dde47b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/441020
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Auto-Submit: Paul Berry <paulberry@google.com>

https://dart.googlesource.com/sdk/+/ac8c7b56cad4d0dd1dc55b4047929afc74295b9e

DEPS

diff --git a/DEPS b/DEPS
index 2164c13..243c7f8 100644
--- a/DEPS
+++ b/DEPS
@@ -229,7 +229,7 @@
 ]
 
 deps = {
-  'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + 'b893b17c18c7d2368d8b3c418dd1396ad8dd5be0',
+  'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + 'ac8c7b56cad4d0dd1dc55b4047929afc74295b9e',
 
   'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + 'd2534e118d18a22b376abe46b2fbac75af12d922',
 

commits.json

diff --git a/commits.json b/commits.json
index 51ff4a3..33aead7 100644
--- a/commits.json
+++ b/commits.json
@@ -1,4 +1,4 @@
 {
   "flutter":"d2534e118d18a22b376abe46b2fbac75af12d922",
-  "flutter/engine/src/flutter/third_party/dart":"b893b17c18c7d2368d8b3c418dd1396ad8dd5be0"
+  "flutter/engine/src/flutter/third_party/dart":"ac8c7b56cad4d0dd1dc55b4047929afc74295b9e"
 }
\ No newline at end of file