Enable Gradle Workflow on Windows (#8201)

* Enable Gradle Workflow on Windows

With this the app created by `flutter create` now compiles on Windows.

* Move OS check to gradle file

packages/flutter_tools/gradle/flutter.gradle

@@ -2,6 +2,7 @@ import java.nio.file.Path
 import java.nio.file.Paths
 
 import com.android.builder.model.AndroidProject
+import org.apache.tools.ant.taskdefs.condition.Os
 import org.gradle.api.DefaultTask
 import org.gradle.api.GradleException
 import org.gradle.api.Project
@@ -28,6 +29,7 @@ apply plugin: FlutterPlugin
 
 class FlutterPlugin implements Plugin<Project> {
     private File flutterRoot
+    private File flutterExecutable
     private String localEngine
     private Properties localProperties
 
@@ -70,6 +72,9 @@ class FlutterPlugin implements Plugin<Project> {
             throw new GradleException("flutter.sdk must point to the Flutter SDK directory")
         }
 
+        String flutterExecutableName = Os.isFamily(Os.FAMILY_WINDOWS) ? "flutter.bat" : "flutter"
+        flutterExecutable = Paths.get(flutterRoot.absolutePath, "bin", flutterExecutableName).toFile();
+
         String flutterJarPath = localProperties.getProperty("flutter.jar")
         if (flutterJarPath != null) {
             File flutterJar = project.file(flutterJarPath)
@@ -86,7 +91,7 @@ class FlutterPlugin implements Plugin<Project> {
             File releaseFlutterJar = baseEnginePath.resolve("android-arm-release").resolve("flutter.jar").toFile()
             if (!debugFlutterJar.isFile()) {
                 project.exec {
-                    executable "${flutterRoot}/bin/flutter"
+                    executable flutterExecutable.absolutePath
                     args "precache"
                 }
                 if (!debugFlutterJar.isFile()) {
@@ -139,6 +144,7 @@ class FlutterPlugin implements Plugin<Project> {
 
             FlutterTask flutterTask = project.tasks.create("flutterBuild${variant.name.capitalize()}", FlutterTask) {
                 flutterRoot this.flutterRoot
+                flutterExecutable this.flutterExecutable
                 buildMode variant.name
                 localEngine this.localEngine
                 targetPath target
@@ -164,6 +170,7 @@ class FlutterExtension {
 
 class FlutterTask extends DefaultTask {
     File flutterRoot
+    File flutterExecutable
     String buildMode
     String localEngine
     String targetPath
@@ -200,7 +207,7 @@ class FlutterTask extends DefaultTask {
 
         if (buildMode != "debug") {
             project.exec {
-                executable "${flutterRoot}/bin/flutter"
+                executable flutterExecutable.absolutePath
                 workingDir sourceDir
                 if (localEngine != null) {
                     args "--local-engine", localEngine
@@ -214,7 +221,7 @@ class FlutterTask extends DefaultTask {
         }
 
         project.exec {
-            executable "${flutterRoot}/bin/flutter"
+            executable flutterExecutable.absolutePath
             workingDir sourceDir
             if (localEngine != null) {
                 args "--local-engine", localEngine

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

@@ -8,6 +8,7 @@ import '../base/common.dart';
 import '../base/file_system.dart';
 import '../base/logger.dart';
 import '../base/os.dart';
+import '../base/platform.dart';
 import '../base/process.dart';
 import '../base/utils.dart';
 import '../build_info.dart';
@@ -176,8 +177,8 @@ Future<Null> buildGradleProject(BuildMode buildMode) async {
   File localProperties = fs.file('android/local.properties');
   if (!localProperties.existsSync()) {
     localProperties.writeAsStringSync(
-        'sdk.dir=${androidSdk.directory}\n'
-        'flutter.sdk=${Cache.flutterRoot}\n'
+        'sdk.dir=${_escapePath(androidSdk.directory)}\n'
+        'flutter.sdk=${_escapePath(Cache.flutterRoot)}\n'
     );
   }
   // Update the local.properties file with the build mode.
@@ -203,6 +204,8 @@ Future<Null> buildGradleProject(BuildMode buildMode) async {
   }
 }
 
+String _escapePath(String path) => platform.isWindows ? path.replaceAll('\\', '\\\\') : path;
+
 Future<Null> buildGradleProjectV1(String gradlew) async {
   // Run 'gradlew build'.
   Status status = logger.startProgress('Running \'gradlew build\'...', expectSlowOperation: true);