Handle missing flutter_test dependency cleanly (#7421)

We now produce a more reasonable error message when we're missing the
flutter_test dependency in a test. Also, remove the flutter_tools stack traces
when the engine dies.

Fixes #6187

dev/missing_dependency_tests/.gitignore

+.atom
+.DS_Store
+.buildlog
+.idea
+.packages
+.pub/
+build/
+packages
+pubspec.lock

dev/missing_dependency_tests/pubspec.yaml

+name: missing_dependency_tests
+dependencies:
+  flutter:
+    sdk: flutter

dev/missing_dependency_tests/trivial_expectation.txt

+<<skip until matching line>>
+<<stderr>>
+<<skip until matching line>>
+Failed to load test harness\. +Are you missing a dependency on flutter_test\?
\ No newline at end of file

dev/missing_dependency_tests/trivial_test.dart

+// Copyright 2017 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 'package:test/test.dart';
+
+void main() {
+  test('Trival test', () {
+    expect(42, 42);
+  });
+}

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

@@ -105,7 +105,7 @@ class FlutterPlatform extends PlatformPlugin {
         subprocessActive = false;
         if (!controllerSinkClosed && exitCode != 0) {
           String message = _getErrorMessage(_getExitCodeMessage(exitCode, 'after tests finished'), testPath, shellPath);
-          controller.sink.addError(new Exception(message));
+          controller.sink.addError(message);
         }
       });
 
@@ -127,13 +127,13 @@ class FlutterPlatform extends PlatformPlugin {
         case _InitialResult.crashed:
           int exitCode = await process.exitCode;
           String message = _getErrorMessage(_getExitCodeMessage(exitCode, 'before connecting to test harness'), testPath, shellPath);
-          controller.sink.addError(new Exception(message));
+          controller.sink.addError(message);
           controller.sink.close();
           await controller.sink.done;
           break;
         case _InitialResult.timedOut:
           String message = _getErrorMessage('Test never connected to test harness.', testPath, shellPath);
-          controller.sink.addError(new Exception(message));
+          controller.sink.addError(message);
           controller.sink.close();
           await controller.sink.done;
           break;
@@ -169,7 +169,7 @@ class FlutterPlatform extends PlatformPlugin {
               int exitCode = await process.exitCode;
               subprocessActive = false;
               String message = _getErrorMessage(_getExitCodeMessage(exitCode, 'before test harness closed its WebSocket'), testPath, shellPath);
-              controller.sink.addError(new Exception(message));
+              controller.sink.addError(message);
               controller.sink.close();
               await controller.sink.done;
               break;
@@ -214,8 +214,12 @@ class FlutterPlatform extends PlatformPlugin {
 import 'dart:convert';
 import 'dart:io'; // ignore: dart_io_import
 
-import 'package:stream_channel/stream_channel.dart';
+// We import this library first in order to trigger an import error for
+// package:test (rather than package:stream_channel) when the developer forgets
+// to add a dependency on package:test.
 import 'package:test/src/runner/plugin/remote_platform_helpers.dart';
+
+import 'package:stream_channel/stream_channel.dart';
 import 'package:test/src/runner/vm/catch_isolate_errors.dart';
 
 import '$testUrl' as test;
@@ -285,7 +289,9 @@ void main() {
       stream.transform(UTF8.decoder)
         .transform(const LineSplitter())
         .listen((String line) {
-          if (line != null)
+          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');
         });
     }

packages/flutter_tools/test/test_test.dart

@@ -17,23 +17,30 @@ import 'src/context.dart';
 
 void main() {
   group('test', () {
+    final String automatedTestsDirectory = path.join('..', '..', 'dev', 'automated_tests');
+    final String flutterTestDirectory = path.join(automatedTestsDirectory, 'flutter_test');
+
     testUsingContext('TestAsyncUtils guarded function test', () async {
       Cache.flutterRoot = '../..';
-      return _testFile('test_async_utils_guarded', 1);
+      return _testFile('test_async_utils_guarded', 1, automatedTestsDirectory, flutterTestDirectory);
     });
     testUsingContext('TestAsyncUtils unguarded function test', () async {
       Cache.flutterRoot = '../..';
-      return _testFile('test_async_utils_unguarded', 1);
+      return _testFile('test_async_utils_unguarded', 1, automatedTestsDirectory, flutterTestDirectory);
+    });
+    testUsingContext('Missing flutter_test dependency', () async {
+      final String missingDependencyTests = path.join('..', '..', 'dev', 'missing_dependency_tests');
+      Cache.flutterRoot = '../..';
+      return _testFile('trivial', 1, missingDependencyTests, missingDependencyTests);
     });
   }, timeout: new Timeout(const Duration(seconds: 5)));
 }
 
-Future<Null> _testFile(String testName, int wantedExitCode) async {
-  final String manualTestsDirectory = path.join('..', '..', 'dev', 'automated_tests');
-  final String fullTestName = path.join(manualTestsDirectory, 'flutter_test', '${testName}_test.dart');
+Future<Null> _testFile(String testName, int wantedExitCode, String workingDirectory, String testDirectory) async {
+  final String fullTestName = path.join(testDirectory, '${testName}_test.dart');
   final File testFile = fs.file(fullTestName);
   expect(testFile.existsSync(), true);
-  final String fullTestExpectation = path.join(manualTestsDirectory, 'flutter_test', '${testName}_expectation.txt');
+  final String fullTestExpectation = path.join(testDirectory, '${testName}_expectation.txt');
   final File expectationFile = fs.file(fullTestExpectation);
   expect(expectationFile.existsSync(), true);
   final ProcessResult exec = await Process.run(
@@ -44,14 +51,17 @@ Future<Null> _testFile(String testName, int wantedExitCode) async {
       '--no-color',
       fullTestName
     ],
-    workingDirectory: manualTestsDirectory
+    workingDirectory: workingDirectory
   );
   expect(exec.exitCode, wantedExitCode);
   final List<String> output = exec.stdout.split('\n');
+  output.add('<<stderr>>');
+  output.addAll(exec.stderr.split('\n'));
   final List<String> expectations = fs.file(fullTestExpectation).readAsLinesSync();
   bool allowSkip = false;
   int expectationLineNumber = 0;
   int outputLineNumber = 0;
+  bool haveSeenStdErrMarker = false;
   while (expectationLineNumber < expectations.length) {
     expect(output, hasLength(greaterThan(outputLineNumber)));
     final String expectationLine = expectations[expectationLineNumber];
@@ -68,10 +78,15 @@ Future<Null> _testFile(String testName, int wantedExitCode) async {
       }
       allowSkip = false;
     }
+    if (expectationLine == '<<stderr>>') {
+      expect(haveSeenStdErrMarker, isFalse);
+      haveSeenStdErrMarker = true;
+    }
     expect(outputLine, matches(expectationLine));
     expectationLineNumber += 1;
     outputLineNumber += 1;
   }
   expect(allowSkip, isFalse);
+  if (!haveSeenStdErrMarker)
     expect(exec.stderr, '');
 }