Fix sky_shell test flakiness caused by CPU throttling (#7543)

When a machine is heavily loaded, it can severely delay the time it
takes for the OS to start a process once it's asked to do so. Our
5 second timeout that we were giving the test process to connect
to the test harness seemed like plenty of time, were it not for the
fact that the test process itself was not being started in time when
CPU throttling was in effect.

This change updates the test timeout to begin counting only once the
test process has been started. We keep the original timeout in play
in the event that the test process *never* starts up for some reason,
but we up that timeout value to 5 minutes.

packages/flutter_tools/lib/src/test/flutter_platform.dart

@@ -20,7 +20,25 @@ import '../dart/package_map.dart';
 import '../globals.dart';
 import 'coverage_collector.dart';
 
+/// The timeout we give the test process to connect to the test harness
+/// once the process has entered its main method.
 const Duration _kTestStartupTimeout = const Duration(seconds: 5);
+
+/// The timeout we give the test process to start executing Dart code. When the
+/// CPU is under severe load, this can take a while, but it's not indicative of
+/// any problem with Flutter, so we give it a large timeout.
+const Duration _kTestProcessTimeout = const Duration(minutes: 5);
+
+/// Message logged by the test process to signal that its main method has begun
+/// execution.
+///
+/// The test harness responds by starting the [_kTestStartupTimeout] countdown.
+/// The CPU may be throttled, which can cause a long delay in between when the
+/// process is spawned and when dart code execution begins; we don't want to
+/// hold that against the test.
+const String _kStartTimeoutTimerMessage = 'sky_shell test process has entered main method';
+
+/// The address at which our WebSocket server resides.
 final InternetAddress _kHost = InternetAddress.LOOPBACK_IP_V4;
 
 void installHook({ String shellPath }) {
@@ -115,8 +133,12 @@ class FlutterPlatform extends PlatformPlugin {
         processToKill: process, // This kills the subprocess whether coverage is enabled or not.
       );
 
+      Completer<Null> timeout = new Completer<Null>();
+
       // Pipe stdout and stderr from the subprocess to our printStatus console.
-      _pipeStandardStreamsToConsole(process);
+      _pipeStandardStreamsToConsole(process, startTimeoutTimer: () {
+        new Future<_InitialResult>.delayed(_kTestStartupTimeout, () => timeout.complete());
+      });
 
       // At this point, three things can happen next:
       // The engine could crash, in which case process.exitCode will complete.
@@ -124,9 +146,10 @@ class FlutterPlatform extends PlatformPlugin {
       // The local test harness could get bored of us.
 
       _InitialResult initialResult = await Future.any(<Future<_InitialResult>>[
-        process.exitCode.then<_InitialResult>((int exitCode) { return _InitialResult.crashed; }),
-        new Future<_InitialResult>.delayed(_kTestStartupTimeout, () { return _InitialResult.timedOut; }),
-        webSocket.future.then<_InitialResult>((WebSocket webSocket) { return _InitialResult.connected; }),
+        process.exitCode.then<_InitialResult>((int exitCode) => _InitialResult.crashed),
+        timeout.future.then<_InitialResult>((Null _) => _InitialResult.timedOut),
+        new Future<_InitialResult>.delayed(_kTestProcessTimeout, () => _InitialResult.timedOut),
+        webSocket.future.then<_InitialResult>((WebSocket webSocket) => _InitialResult.connected),
       ]);
 
       switch (initialResult) {
@@ -227,6 +250,7 @@ import 'package:test/src/runner/vm/catch_isolate_errors.dart';
 import '$testUrl' as test;
 
 void main() {
+  print('$_kStartTimeoutTimerMessage');
   String server = Uri.decodeComponent('$encodedWebsocketUrl');
   StreamChannel channel = serializeSuite(() {
     catchIsolateErrors();
@@ -285,13 +309,18 @@ void main() {
     return processManager.start(executable, arguments, environment: environment);
   }
 
-  void _pipeStandardStreamsToConsole(Process process) {
+  void _pipeStandardStreamsToConsole(
+    Process process, {
+    void startTimeoutTimer(),
+  }) {
     for (Stream<List<int>> stream in
         <Stream<List<int>>>[process.stderr, process.stdout]) {
       stream.transform(UTF8.decoder)
         .transform(const LineSplitter())
         .listen((String line) {
-          if (line.startsWith('error: Unable to read Dart source \'package:test/'))
+          if (line == _kStartTimeoutTimerMessage && startTimeoutTimer != null)
+            startTimeoutTimer();
+          else if (line.startsWith('error: Unable to read Dart source \'package:test/'))
             printError('\n\nFailed to load test harness. Are you missing a dependency on flutter_test?\n');
           else if (line != null)
             printStatus('Shell: $line');