First run simulator with -n to ensure it always opens a device (#17460)

packages/flutter_tools/lib/src/android/android_emulator.dart

@@ -41,20 +41,22 @@ class AndroidEmulator extends Emulator {
   String get label => _properties['avd.ini.displayname'];
 
   @override
-  Future<void> launch() async {
-    final Future<void> launchResult =
+  Future<bool> launch() async {
+    final Future<bool> launchResult =
         runAsync(<String>[getEmulatorPath(), '-avd', id])
             .then((RunResult runResult) {
               if (runResult.exitCode != 0) {
                 printError('$runResult');
+                return false;
               }
+              return true;
             });
     // emulator continues running on a successful launch so if we
     // haven't quit within 3 seconds we assume that's a success and just
     // return.
-    await Future.any<void>(<Future<void>>[
+    return Future.any<bool>(<Future<bool>>[
       launchResult,
-      new Future<void>.delayed(const Duration(seconds: 3))
+      new Future<void>.delayed(const Duration(seconds: 3)).then((_) => true)
     ]);
   }
 }

packages/flutter_tools/lib/src/emulator.dart

@@ -95,7 +95,7 @@ abstract class Emulator {
     return id == other.id;
   }
 
-  Future<void> launch();
+  Future<bool> launch();
 
   @override
   String toString() => name;

packages/flutter_tools/lib/src/ios/ios_emulators.dart

@@ -36,12 +36,27 @@ class IOSEmulator extends Emulator {
   String get label => null;
 
   @override
-  Future<void> launch() async {
-    final RunResult launchResult =
-        await runAsync(<String>['open', '-a', getSimulatorPath()]);
-    if (launchResult.exitCode != 0) {
-      printError('$launchResult');
+  Future<bool> launch() async {
+    Future<bool> launchSimulator(List<String> additionalArgs) async {
+      final List<String> args = <String>['open']
+          .followedBy(additionalArgs)
+          .followedBy(<String>['-a', getSimulatorPath()]);
+
+      final RunResult launchResult = await runAsync(args);
+      if (launchResult.exitCode != 0) {
+        printError('$launchResult');
+        return false;
+      }
+      return true;
     }
+
+    // First run with `-n` to force a device to boot if there isn't already one
+    if (!await launchSimulator(<String>['-n']))
+      return false;
+    
+    // Run again to force it to Foreground (using -n doesn't force existing
+    // devices to the foreground)
+    return launchSimulator(<String>[]);
   }
 }
 

packages/flutter_tools/test/emulator_test.dart

@@ -62,7 +62,7 @@ class _MockEmulator extends Emulator {
   final String label;
 
   @override
-  Future<void> launch() {
+  Future<bool> launch() {
     throw new UnimplementedError('Not implemented in Mock');
   }
 }