Add --preview-dart-2 to 'flutter drive' (#14858)

* Add --preview-dart-2 to 'flutter drive'

* Run driver side in --preview-dart-2 mode.

* Fix test

packages/flutter_tools/lib/src/commands/drive.dart

@@ -73,6 +73,10 @@ class DriveCommand extends RunCommandBase {
       valueHelp:
         'path'
     );
+
+    argParser.addFlag('preview-dart-2',
+        defaultsTo: false,
+        help: 'Preview Dart 2.0 functionality.');
   }
 
   @override
@@ -130,7 +134,7 @@ class DriveCommand extends RunCommandBase {
     Cache.releaseLockEarly();
 
     try {
-      await testRunner(<String>[testFile], observatoryUri);
+      await testRunner(<String>[testFile], observatoryUri, argResults['preview-dart-2']);
     } catch (error, stackTrace) {
       if (error is ToolExit)
         rethrow;
@@ -282,19 +286,23 @@ Future<LaunchResult> _startApp(DriveCommand command) async {
 }
 
 /// Runs driver tests.
-typedef Future<Null> TestRunner(List<String> testArgs, String observatoryUri);
+typedef Future<Null> TestRunner(List<String> testArgs, String observatoryUri, bool previewDart2);
 TestRunner testRunner = _runTests;
 void restoreTestRunner() {
   testRunner = _runTests;
 }
 
-Future<Null> _runTests(List<String> testArgs, String observatoryUri) async {
+Future<Null> _runTests(List<String> testArgs, String observatoryUri, bool previewDart2) async {
   printTrace('Running driver tests.');
 
   PackageMap.globalPackagesPath = fs.path.normalize(fs.path.absolute(PackageMap.globalPackagesPath));
   final List<String> args = testArgs.toList()
     ..add('--packages=${PackageMap.globalPackagesPath}')
     ..add('-rexpanded');
+  if (previewDart2) {
+    args.add('--preview-dart-2');
+  }
+
   final String dartVmPath = fs.path.join(dartSdkPath, 'bin', 'dart');
   final int result = await runCommandAndStreamOutput(
     <String>[dartVmPath]..addAll(dartVmFlags)..addAll(args),

packages/flutter_tools/test/commands/drive_test.dart

@@ -54,7 +54,7 @@ void main() {
       appStarter = (DriveCommand command) {
         throw 'Unexpected call to appStarter';
       };
-      testRunner = (List<String> testArgs, String observatoryUri) {
+      testRunner = (List<String> testArgs, String observatoryUri, bool previewDart2) {
         throw 'Unexpected call to testRunner';
       };
       appStopper = (DriveCommand command) {
@@ -169,7 +169,7 @@ void main() {
       appStarter = expectAsync1((DriveCommand command) async {
         return new LaunchResult.succeeded();
       });
-      testRunner = expectAsync2((List<String> testArgs, String observatoryUri) async {
+      testRunner = expectAsync3((List<String> testArgs, String observatoryUri, bool previewDart2) async {
         expect(testArgs, <String>[testFile]);
         return null;
       });
@@ -200,7 +200,7 @@ void main() {
       appStarter = expectAsync1((DriveCommand command) async {
         return new LaunchResult.succeeded();
       });
-      testRunner = (List<String> testArgs, String observatoryUri) async {
+      testRunner = (List<String> testArgs, String observatoryUri, bool previewDart2) async {
         throwToolExit(null, exitCode: 123);
       };
       appStopper = expectAsync1((DriveCommand command) async {