ProcessResultMatcher created and used in test (#127414)

Addresses issues:
- https://github.com/flutter/flutter/issues/127135
- https://github.com/flutter/flutter/issues/99767

Creates a matcher class that we can use for `ProcessResult` to check for the exit code.

*List which issues are fixed by this PR. You must list at least one issue.*

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

packages/flutter_tools/test/host_cross_arch.shard/cache_test.dart

@@ -31,6 +31,11 @@ HostPlatform _identifyMacBinaryArch(String path) {
   final ProcessResult result = processManager.runSync(
     <String>['file', _dartBinary.path],
   );
+  expect(
+      result,
+      ProcessResultMatcher(
+        stdoutSubstring: '${_dartBinary.path}: Mach-O 64-bit executable',
+      ));
   final RegExpMatch? match = pattern.firstMatch(result.stdout as String);
   if (match == null) {
     fail('Unrecognized STDOUT from `file`: "${result.stdout}"');

packages/flutter_tools/test/integration.shard/gradle_non_android_plugin_test.dart

@@ -4,6 +4,7 @@
 
 import 'package:file/file.dart';
 import 'package:file_testing/file_testing.dart';
+import 'package:flutter_tools/src/base/io.dart';
 
 import '../src/common.dart';
 import 'test_utils.dart';
@@ -68,7 +69,7 @@ void main() {
     // Build example APK
     final Directory exampleDir = projectRoot.childDirectory('example');
 
-    processManager.runSync(<String>[
+    final ProcessResult result = processManager.runSync(<String>[
       flutterBin,
       ...getLocalEngineArguments(),
       'build',
@@ -78,6 +79,8 @@ void main() {
       '--verbose',
     ], workingDirectory: exampleDir.path);
 
+    expect(result, ProcessResultMatcher());
+
     final String exampleAppApk = fileSystem.path.join(
       exampleDir.path,
       'build',

packages/flutter_tools/test/integration.shard/test_utils.dart

@@ -124,3 +124,82 @@ abstract final class AppleTestUtils {
     return nmOutput.isEmpty ? const <String>[] : nmOutput.split('\n');
   }
 }
+
+/// Matcher to be used for [ProcessResult] returned
+/// from a process run
+///
+/// The default for [expectedExitCode] will be 0 while
+/// [stdoutSubstring] and [stderrSubstring] are both optional
+class ProcessResultMatcher extends Matcher {
+  ProcessResultMatcher({
+    this.expectedExitCode = 0,
+    this.stdoutSubstring,
+    this.stderrSubstring,
+  });
+
+  /// The expected exit code to get returned from a process run
+  final int expectedExitCode;
+
+  /// Substring to find in the process's stdout
+  final String? stdoutSubstring;
+
+  /// Substring to find in the process's stderr
+  final String? stderrSubstring;
+
+  bool _foundStdout = true;
+  bool _foundStderr = true;
+
+  @override
+  Description describe(Description description) {
+    description.add('a process with exit code $expectedExitCode');
+    if (stdoutSubstring != null) {
+      description.add(' and stdout: "$stdoutSubstring"');
+    }
+    if (stderrSubstring != null) {
+      description.add(' and stderr: "$stderrSubstring"');
+    }
+
+    return description;
+  }
+
+  @override
+  bool matches(dynamic item, Map<dynamic, dynamic> matchState) {
+    final ProcessResult result = item as ProcessResult;
+
+    if (stdoutSubstring != null) {
+      _foundStdout = (result.stdout as String).contains(stdoutSubstring!);
+      matchState['stdout'] = result.stdout;
+    }
+
+    if (stderrSubstring != null) {
+      _foundStderr = (result.stderr as String).contains(stderrSubstring!);
+      matchState['stderr'] = result.stderr;
+    }
+
+    return result.exitCode == expectedExitCode && _foundStdout && _foundStderr;
+  }
+
+  @override
+  Description describeMismatch(
+    Object? item,
+    Description mismatchDescription,
+    Map<dynamic, dynamic> matchState,
+    bool verbose,
+  ) {
+    final ProcessResult result = item! as ProcessResult;
+
+    if (result.exitCode != expectedExitCode) {
+      mismatchDescription.add('Actual exitCode was ${result.exitCode}');
+    }
+
+    if (matchState.containsKey('stdout')) {
+      mismatchDescription.add('Actual stdout:\n${matchState["stdout"]}');
+    }
+
+    if (matchState.containsKey('stderr')) {
+      mismatchDescription.add('Actual stderr:\n${matchState["stderr"]}');
+    }
+
+    return mismatchDescription;
+  }
+}