Add build-after-edit benchmark (#107192)

* Add build-after-edit benchmark

* Clean up analyzer warnings

* Restore line break

Author: aam

dev/devicelab/lib/tasks/perf_tests.dart

@@ -1335,6 +1335,21 @@ class CompileTest {
         ...compileSecondDebug,
       };
 
+      final File mainDart = File('$testDirectory/lib/main.dart');
+      if (mainDart.existsSync()) {
+        final List<int> bytes = mainDart.readAsBytesSync();
+        // "Touch" the file
+        mainDart.writeAsStringSync(' ', mode: FileMode.append, flush: true);
+        // Build after "edit" without clean should be faster than first build
+        final Map<String, dynamic> compileAfterEditDebug = await _compileDebug(
+          clean: false,
+          metricKey: 'debug_compile_after_edit_millis',
+        );
+        metrics.addAll(compileAfterEditDebug);
+        // Revert the changes
+        mainDart.writeAsBytesSync(bytes, flush: true);
+      }
+
       return TaskResult.success(metrics, benchmarkScoreKeys: metrics.keys.toList());
     });
   }