Add debug_second_compile_millis benchmark metric (#77420)

dev/devicelab/lib/tasks/perf_tests.dart

@@ -1000,9 +1000,21 @@ class CompileTest {
       await device.unlock();
       await flutter('packages', options: <String>['get']);
 
+      final Map<String, dynamic> compileRelease = await _compileApp(reportPackageContentSizes: reportPackageContentSizes);
+      final Map<String, dynamic> compileDebug = await _compileDebug(
+        clean: true,
+        metricKey: 'debug_full_compile_millis',
+      );
+      // Build again without cleaning, should be faster.
+      final Map<String, dynamic> compileSecondDebug = await _compileDebug(
+        clean: false,
+        metricKey: 'debug_second_compile_millis',
+      );
+
       final Map<String, dynamic> metrics = <String, dynamic>{
-        ...await _compileApp(reportPackageContentSizes: reportPackageContentSizes),
-        ...await _compileDebug(),
+        ...compileRelease,
+        ...compileDebug,
+        ...compileSecondDebug,
       };
 
       return TaskResult.success(metrics, benchmarkScoreKeys: metrics.keys.toList());
@@ -1082,8 +1094,13 @@ class CompileTest {
     return metrics;
   }
 
-  static Future<Map<String, dynamic>> _compileDebug() async {
-    await flutter('clean');
+  static Future<Map<String, dynamic>> _compileDebug({
+    @required bool clean,
+    @required String metricKey,
+  }) async {
+    if (clean) {
+      await flutter('clean');
+    }
     final Stopwatch watch = Stopwatch();
     final List<String> options = <String>['--debug'];
     switch (deviceOperatingSystem) {
@@ -1109,7 +1126,7 @@ class CompileTest {
     watch.stop();
 
     return <String, dynamic>{
-      'debug_full_compile_millis': watch.elapsedMilliseconds,
+      metricKey: watch.elapsedMilliseconds,
     };
   }