test(dart-sentry): add compilation tests for example_web

Added tests to check if example_web project can be compiled with
build_runner.

Resolves #1893

Author: daniel-v

dart/test/example_web_compile_test.dart

@@ -0,0 +1,74 @@
+@TestOn('vm')
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:test/test.dart';
+
+// Tests for the following issue
+// https://github.com/getsentry/sentry-dart/issues/1893
+void main() {
+  group('Compile example_web', () {
+    test('dart pub get should run successfully', () async {
+      final result = await _runProcess('dart pub get',
+          workingDirectory: _exampleWebWorkingDir);
+      expect(result.exitCode, 0,
+          reason: 'Could run `dart pub get` for example_web. '
+              'Likely caused by outdated dependencies');
+    });
+    test('dart run build_runner build should run successfully', () async {
+      // running this test locally require clean working directory
+      final cleanResult = await _runProcess('dart run build_runner clean',
+          workingDirectory: _exampleWebWorkingDir);
+      expect(cleanResult.exitCode, 0);
+      final result = await _runProcess(
+          'dart run build_runner build -r web -o build --delete-conflicting-outputs',
+          workingDirectory: _exampleWebWorkingDir);
+      expect(result.exitCode, 0,
+          reason: 'Could not compile example_web project');
+      expect(
+          result.stdout,
+          isNot(contains(
+              'Skipping compiling sentry_dart_web_example|web/main.dart')),
+          reason:
+              'Could not compile main.dart, likely because of dart:io import.');
+      expect(result.stdout,
+          contains('build_web_compilers:entrypoint on web/main.dart:Compiled'));
+    });
+  });
+}
+
+/// Runs [command] with command's stdout and stderr being forwrarded to
+/// test runner's respective streams. It buffers stdout and returns it.
+///
+/// Returns [_CommandResult] with exitCode and stdout as a single sting
+Future<_CommandResult> _runProcess(String command,
+    {String workingDirectory = '.'}) async {
+  final parts = command.split(' ');
+  assert(parts.isNotEmpty);
+  final cmd = parts[0];
+  final args = parts.skip(1).toList();
+  final process =
+      await Process.start(cmd, args, workingDirectory: workingDirectory);
+  // forward standard streams
+  unawaited(stderr.addStream(process.stderr));
+  final buffer = <int>[];
+  await for (final units in process.stdout) {
+    buffer.addAll(units);
+    stdout.add(units);
+  }
+  final processOut = utf8.decode(buffer);
+  int exitCode = await process.exitCode;
+  return _CommandResult(exitCode: exitCode, stdout: processOut);
+}
+
+String get _exampleWebWorkingDir {
+  return Directory.current.uri.resolve('./example_web').normalizePath().path;
+}
+
+class _CommandResult {
+  final int exitCode;
+  final String stdout;
+
+  const _CommandResult({required this.exitCode, required this.stdout});
+}