[flutter_plugin_tools] Fix build-examples for packages (#4248)

The build-examples command was filtering what it attempted to build by plugin platform, which means it never does anything for non-plugin packages. flutter/packages has steps that run this command, which suggests it used to work and regressed at some point, but nobody noticed; this will re-enable those builds so that we are getting CI coverage that the examples in flutter/packages build.

Mostly fixes https://github.com/flutter/flutter/issues/88435 (needs a flutter/packages tool pin roll to pick this up)

script/tool/test/build_examples_command_test.dart

@@ -82,6 +82,35 @@ void main() {
           ]));
     });
 
+    test('fails if a plugin has no examples', () async {
+      createFakePlugin('plugin', packagesDir,
+          examples: <String>[],
+          platformSupport: <String, PlatformDetails>{
+            kPlatformIos: const PlatformDetails(PlatformSupport.inline)
+          });
+
+      processRunner
+              .mockProcessesForExecutable[getFlutterCommand(mockPlatform)] =
+          <io.Process>[
+        MockProcess(exitCode: 1) // flutter packages get
+      ];
+
+      Error? commandError;
+      final List<String> output = await runCapturingPrint(
+          runner, <String>['build-examples', '--ios'], errorHandler: (Error e) {
+        commandError = e;
+      });
+
+      expect(commandError, isA<ToolExit>());
+      expect(
+          output,
+          containsAllInOrder(<Matcher>[
+            contains('The following packages had errors:'),
+            contains('  plugin:\n'
+                '    No examples found'),
+          ]));
+    });
+
     test('building for iOS when plugin is not set up for iOS results in no-op',
         () async {
       mockPlatform.isMacOS = true;
@@ -517,5 +546,138 @@ void main() {
                 pluginExampleDirectory.path),
           ]));
     });
+
+    test('logs skipped platforms', () async {
+      createFakePlugin('plugin', packagesDir,
+          platformSupport: <String, PlatformDetails>{
+            kPlatformAndroid: const PlatformDetails(PlatformSupport.inline),
+          });
+
+      final List<String> output = await runCapturingPrint(
+          runner, <String>['build-examples', '--apk', '--ios', '--macos']);
+
+      expect(
+        output,
+        containsAllInOrder(<Matcher>[
+          contains('Skipping unsupported platform(s): iOS, macOS'),
+        ]),
+      );
+    });
+
+    group('packages', () {
+      test('builds when requested platform is supported by example', () async {
+        final Directory packageDirectory = createFakePackage(
+            'package', packagesDir, isFlutter: true, extraFiles: <String>[
+          'example/ios/Runner.xcodeproj/project.pbxproj'
+        ]);
+
+        final List<String> output = await runCapturingPrint(
+            runner, <String>['build-examples', '--ios']);
+
+        expect(
+          output,
+          containsAllInOrder(<Matcher>[
+            contains('Running for package'),
+            contains('BUILDING package/example for iOS'),
+          ]),
+        );
+
+        expect(
+            processRunner.recordedCalls,
+            orderedEquals(<ProcessCall>[
+              ProcessCall(
+                  getFlutterCommand(mockPlatform),
+                  const <String>[
+                    'build',
+                    'ios',
+                    '--no-codesign',
+                  ],
+                  packageDirectory.childDirectory('example').path),
+            ]));
+      });
+
+      test('skips non-Flutter examples', () async {
+        createFakePackage('package', packagesDir, isFlutter: false);
+
+        final List<String> output = await runCapturingPrint(
+            runner, <String>['build-examples', '--ios']);
+
+        expect(
+          output,
+          containsAllInOrder(<Matcher>[
+            contains('Running for package'),
+            contains('No examples found supporting requested platform(s).'),
+          ]),
+        );
+
+        expect(processRunner.recordedCalls, orderedEquals(<ProcessCall>[]));
+      });
+
+      test('skips when there is no example', () async {
+        createFakePackage('package', packagesDir,
+            isFlutter: true, examples: <String>[]);
+
+        final List<String> output = await runCapturingPrint(
+            runner, <String>['build-examples', '--ios']);
+
+        expect(
+          output,
+          containsAllInOrder(<Matcher>[
+            contains('Running for package'),
+            contains('No examples found supporting requested platform(s).'),
+          ]),
+        );
+
+        expect(processRunner.recordedCalls, orderedEquals(<ProcessCall>[]));
+      });
+
+      test('skip when example does not support requested platform', () async {
+        createFakePackage('package', packagesDir,
+            isFlutter: true,
+            extraFiles: <String>['example/linux/CMakeLists.txt']);
+
+        final List<String> output = await runCapturingPrint(
+            runner, <String>['build-examples', '--ios']);
+
+        expect(
+          output,
+          containsAllInOrder(<Matcher>[
+            contains('Running for package'),
+            contains('Skipping iOS for package/example; not supported.'),
+            contains('No examples found supporting requested platform(s).'),
+          ]),
+        );
+
+        expect(processRunner.recordedCalls, orderedEquals(<ProcessCall>[]));
+      });
+
+      test('logs skipped platforms when only some are supported', () async {
+        final Directory packageDirectory = createFakePackage(
+            'package', packagesDir,
+            isFlutter: true,
+            extraFiles: <String>['example/linux/CMakeLists.txt']);
+
+        final List<String> output = await runCapturingPrint(
+            runner, <String>['build-examples', '--apk', '--linux']);
+
+        expect(
+          output,
+          containsAllInOrder(<Matcher>[
+            contains('Running for package'),
+            contains('Building for: Android, Linux'),
+            contains('Skipping Android for package/example; not supported.'),
+          ]),
+        );
+
+        expect(
+            processRunner.recordedCalls,
+            orderedEquals(<ProcessCall>[
+              ProcessCall(
+                  getFlutterCommand(mockPlatform),
+                  const <String>['build', 'linux'],
+                  packageDirectory.childDirectory('example').path),
+            ]));
+      });
+    });
   });
 }