Print warning if flutter drive is run in debug (#30747)

## Description

Print actionable warnings if `flutter drive` (where most of our performance benchmarks come from) is run in debug mode and it tries to gather benchmarks using `traceAction`.

## Related Issues

https://github.com/flutter/flutter/issues/30625

## Tests

I added the following tests:
* drive_perf_debug_warning devicelab test

dev/devicelab/bin/tasks/drive_perf_debug_warning.dart

+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:async';
+
+import 'package:flutter_devicelab/framework/adb.dart';
+import 'package:flutter_devicelab/framework/framework.dart';
+import 'package:flutter_devicelab/framework/utils.dart';
+
+Future<String> _runWithMode(String mode, String deviceId) {
+  return evalFlutter('drive', options: <String>[
+    mode,
+    '-t',
+    'test_driver/scroll_perf.dart',
+    '-d',
+    deviceId,
+  ]);
+}
+
+Future<TaskResult> run() async {
+  cd('${flutterDirectory.path}/examples/flutter_gallery');
+  final Device device = await devices.workingDevice;
+  await device.unlock();
+  final String deviceId = device.deviceId;
+  await flutter('packages', options: <String>['get']);
+
+  const String warningPiece = 'THIS BENCHMARK IS BEING RUN IN DEBUG MODE';
+
+  final String debugOutput = await _runWithMode('--debug', deviceId);
+  if (!debugOutput.contains(warningPiece)) {
+    return TaskResult.failure(
+        'Could not find the following warning message piece: $warningPiece'
+    );
+  }
+
+  final String profileOutput = await _runWithMode('--profile', deviceId);
+  if (profileOutput.contains(warningPiece)) {
+    return TaskResult.failure(
+      'Unexpected warning message piece in profile mode: $warningPiece'
+    );
+  }
+
+  return TaskResult.success(null);
+}
+
+Future<void> main() async {
+  deviceOperatingSystem = DeviceOperatingSystem.android;
+  await task(run);
+}

dev/devicelab/manifest.yaml

@@ -148,6 +148,13 @@ tasks:
     stage: devicelab
     required_agent_capabilities: ["mac/android"]
 
+  drive_perf_debug_warning:
+    description: >
+      Check that driver will print warnings when traceAction is called in debug mode.
+    stage: devicelab
+    required_agent_capabilities: ["mac/android"]
+    flaky: true
+
   cull_opacity_perf__timeline_summary:
     description: >
       Measures the runtime performance of culling opacity widgets on Android.

packages/flutter_driver/lib/src/driver/driver.dart

@@ -150,6 +150,22 @@ class FlutterDriver {
 
   static int _nextDriverId = 0;
 
+  // The additional blank line in the beginning is for _log.warning.
+  static const String _kDebugWarning = '''
+
+┏╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍┓
+┇ ⚠    THIS BENCHMARK IS BEING RUN IN DEBUG MODE     ⚠  ┇
+┡╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍┦
+│                                                       │
+│  Numbers obtained from a benchmark while asserts are  │
+│  enabled will not accurately reflect the performance  │
+│  that will be experienced by end users using release  ╎
+│  builds. Benchmarks should be run using this command  ┆
+│  line:  flutter drive --profile test_perf.dart        ┊
+│                                                       ┊
+└─────────────────────────────────────────────────╌┄┈  🐢
+''';
+
   /// Connects to a Flutter application.
   ///
   /// Resumes the application if it is currently paused (e.g. at a breakpoint).
@@ -708,7 +724,9 @@ class FlutterDriver {
   /// [getFlagList]: https://github.com/dart-lang/sdk/blob/master/runtime/vm/service/service.md#getflaglist
   Future<List<Map<String, dynamic>>> getVmFlags() async {
     final Map<String, dynamic> result = await _peer.sendRequest('getFlagList');
-    return result['flags'];
+    return result != null
+        ? result['flags'].cast<Map<String,dynamic>>()
+        : const <Map<String, dynamic>>[];
   }
 
   /// Starts recording performance traces.
@@ -764,6 +782,16 @@ class FlutterDriver {
     }
   }
 
+  Future<bool> _isPrecompiledMode() async {
+    final List<Map<String, dynamic>> flags = await getVmFlags();
+    for(Map<String, dynamic> flag in flags) {
+      if (flag['name'] == 'precompiled_mode') {
+        return flag['valueAsString'] == 'true';
+      }
+    }
+    return false;
+  }
+
   /// Runs [action] and outputs a performance trace for it.
   ///
   /// Waits for the `Future` returned by [action] to complete prior to stopping
@@ -778,6 +806,9 @@ class FlutterDriver {
   /// If [retainPriorEvents] is true, retains events recorded prior to calling
   /// [action]. Otherwise, prior events are cleared before calling [action]. By
   /// default, prior events are cleared.
+  ///
+  /// If this is run in debug mode, a warning message will be printed to suggest
+  /// running the benchmark in profile mode instead.
   Future<Timeline> traceAction(
     Future<dynamic> action(), {
     List<TimelineStream> streams = _defaultStreams,
@@ -788,6 +819,10 @@ class FlutterDriver {
     }
     await startTracing(streams: streams);
     await action();
+
+    if (!(await _isPrecompiledMode())) {
+      _log.warning(_kDebugWarning);
+    }
     return stopTracingAndDownloadTimeline();
   }