Improve AOT snapshot input verification + cleanup (#17144)

Bugfix: Moves AOT snapshot input verification past where the last input
is added to the inputs list.

Cleanup:
* Extracts _isValidAotPlatform method.
* Moves non-platform-specific logic to the top.
* Moves variable declaration closer to first use, and inlines to a
  narrower scope where possible.

This relands #17136, which was reverted in #17142 due to breakage in
on-device iOS debug builds.

packages/flutter_tools/test/base/build_test.dart

@@ -662,13 +662,12 @@ void main() {
         '--url_mapping=dart:ui,${fs.path.join(skyEnginePath, 'lib', 'ui', 'ui.dart')}',
         '--url_mapping=dart:vmservice_io,${fs.path.join(skyEnginePath, 'sdk_ext', 'vmservice_io.dart')}',
         '--dependencies=${fs.path.join(outputPath, 'snapshot.d')}',
-        '--snapshot_kind=core',
-        'snapshot.dart',
-        '--no-checked',
-        '--conditional_directives',
         '--reify-generic-functions',
         '--strong',
-        'main.dill',
+        '--no-checked',
+        '--conditional_directives',
+        '--snapshot_kind=core',
+        'snapshot.dart',
       ]);
     }, overrides: contextOverrides);
 
@@ -704,14 +703,14 @@ void main() {
         '--url_mapping=dart:ui,${fs.path.join(skyEnginePath, 'lib', 'ui', 'ui.dart')}',
         '--url_mapping=dart:vmservice_io,${fs.path.join(skyEnginePath, 'sdk_ext', 'vmservice_io.dart')}',
         '--dependencies=${fs.path.join(outputPath, 'snapshot.d')}',
+        '--reify-generic-functions',
+        '--strong',
+        '--no-checked',
+        '--conditional_directives',
         '--embedder_entry_points_manifest=$kVmEntrypoints',
         '--embedder_entry_points_manifest=$kIoEntries',
         '--snapshot_kind=app-aot-assembly',
         '--assembly=${fs.path.join(outputPath, 'snapshot_assembly.S')}',
-        '--no-checked',
-        '--conditional_directives',
-        '--reify-generic-functions',
-        '--strong',
         'main.dill',
       ]);
     }, overrides: contextOverrides);
@@ -748,12 +747,12 @@ void main() {
         '--url_mapping=dart:ui,${fs.path.join(skyEnginePath, 'lib', 'ui', 'ui.dart')}',
         '--url_mapping=dart:vmservice_io,${fs.path.join(skyEnginePath, 'sdk_ext', 'vmservice_io.dart')}',
         '--dependencies=${fs.path.join(outputPath, 'snapshot.d')}',
+        '--reify-generic-functions',
+        '--strong',
         '--embedder_entry_points_manifest=$kVmEntrypoints',
         '--embedder_entry_points_manifest=$kIoEntries',
         '--snapshot_kind=app-aot-assembly',
         '--assembly=${fs.path.join(outputPath, 'snapshot_assembly.S')}',
-        '--reify-generic-functions',
-        '--strong',
         'main.dill',
       ]);
     }, overrides: contextOverrides);