Update source_gen v0.8.3+1 -> 0.9.1.

## 0.9.1

* The result of `ConstantReader.revive()` now returns a `Revivable` that assumes
  access to a private class, constructor, or function _instead_ of `null` where
  possible. This allows generators that use `part` files to still use this
  functionality _and_ allows generators that use separate libraries to emit more
  actionable error messages (i.e. `"cannot use private class _X"`).

* `Revivable.isPrivate` now returns `true` when the underyling class was public
  but the constructor was private, or the `Revivable` was pointing to a
  top-level or static private field or method. Previously it was only `true`
  when referencing a private class.

## 0.9.0+1

* Fix `LibraryReader.classElements` to return classes from parts, if they exist,
  as well as from the defining library file.

PiperOrigin-RevId: 212332039

Author: matanlurey

_tests/test/compiler_integration/unrevivable_const_test.dart

@@ -17,9 +17,7 @@ void main() {
       final InjectorFactory example = null;
     ''', errors: [
       allOf([
-        // TODO(matanl): Improve after source_gen#374.
-        // https://github.com/dart-lang/source_gen/issues/374
-        contains('Could not reference const object'),
+        contains('While attempting to resolve a constant value for a provider'),
         contains('TestClass'),
       ]),
     ]);

angular/pubspec.yaml

@@ -29,7 +29,7 @@ dependencies:
   code_builder: '^3.0.1'
   csslib: ^0.14.5
   path: ^1.6.1
-  source_gen: ">=0.8.0 <0.10.0"
+  source_gen: ">=0.9.1 <0.10.0"
   source_span: ^1.4.0
 
 # === vvv REMOVE WHEN PUBLISHING vvv ===

angular_compiler/pubspec.yaml

@@ -17,7 +17,7 @@ dependencies:
   logging: '>=0.9.0 <0.12.0'
   meta: ^1.1.4
   path: ^1.6.1
-  source_gen: ">=0.8.0 <0.10.0"
+  source_gen: ">=0.9.1 <0.10.0"
   source_span: ^1.4.0
   stack_trace: ^1.9.1