Bump collection to c90b19f07b48391f3b1b4c39dd06ef0177f8e07c

Changes:
```
> git log --format="%C(auto) %h %s" 141d83a..c90b19f
 https://dart.googlesource.com/collection.git/+/c90b19f blast_repo fixes (348)
 https://dart.googlesource.com/collection.git/+/a1d2507 Add flattenedToList and flattenedToSet (328)
 https://dart.googlesource.com/collection.git/+/e9219c7 Deprecate whereNotNull from IterableNullableExtensions (332)
 https://dart.googlesource.com/collection.git/+/9e441f1 Deprecate transitiveClosure (336)

```

Diff: https://dart.googlesource.com/collection.git/+/141d83af3d7586ae9d27de610fd426071c98e5d3..c90b19f07b48391f3b1b4c39dd06ef0177f8e07c/
Change-Id: I9ed24345c132ad92ae65c5e70ab37647fad0ec00
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/370960
Reviewed-by: Nate Bosch <nbosch@google.com>
Commit-Queue: Devon Carew <devoncarew@google.com>

pkg/sourcemap_testing/lib/src/stepping_helper.dart

@@ -3,7 +3,6 @@ import 'dart:ffi' show Abi;
 import 'dart:io';
 
 import 'package:_fe_analyzer_shared/src/testing/annotated_code_helper.dart';
-import 'package:collection/collection.dart' show IterableNullableExtension;
 import 'package:dart2js_tools/src/util.dart';
 import 'package:expect/minitest.dart'; // ignore: deprecated_member_use
 import 'package:path/path.dart' as path;
@@ -430,7 +429,7 @@ ProcessResult _runD8(String outInspectorPath, List<String> scriptD8Command,
     ...scriptD8Command,
     '--',
     debugAction,
-    ...breakpoints.whereNotNull()
+    ...breakpoints.nonNulls
   ]);
   if (runResult.exitCode != 0) {
     print(runResult.stderr);

pkg/sourcemap_testing/pubspec.yaml

@@ -4,12 +4,11 @@ name: sourcemap_testing
 publish_to: none
 
 environment:
-  sdk: '>=2.16.0 <3.0.0'
+  sdk: ^3.0.0
 
 # Use 'any' constraints here; we get our versions from the DEPS file.
 dependencies:
   _fe_analyzer_shared: any
-  collection: any
   dart2js_tools: any
   expect: any
   path: any