Expose the engine-src-path flag in the Gradle plugin

examples/hello_android/buildSrc/src/main/groovy/FlutterPlugin.groovy

@@ -18,6 +18,7 @@ import org.gradle.api.tasks.TaskAction
 
 class FlutterPlugin implements Plugin<Project> {
     private File sdkDir
+    private File engineSrcDir
 
     @Override
     void apply(Project project) {
@@ -42,6 +43,14 @@ class FlutterPlugin implements Plugin<Project> {
             throw new GradleException("flutter.sdk must point to the Flutter SDK directory")
         }
 
+        String engineSrcPath = properties.getProperty("flutter.engineSrcPath")
+        if (engineSrcPath != null) {
+            engineSrcDir = project.file(engineSrcPath)
+            if (!engineSrcDir.isDirectory()) {
+                throw new GradleException("flutter.engineSrcPath must be a Flutter engine source directory")
+            }
+        }
+
         project.extensions.create("flutter", FlutterExtension)
         project.dependencies.add("compile", flutterEngine)
         project.afterEvaluate this.&addFlutterTask
@@ -56,6 +65,7 @@ class FlutterPlugin implements Plugin<Project> {
             sdkDir this.sdkDir
             sourceDir project.file(project.flutter.source)
             intermediateDir project.file("${project.buildDir}/${AndroidProject.FD_INTERMEDIATES}/flutter")
+            engineSrcDir this.engineSrcDir
         }
 
         project.android.applicationVariants.all { variant ->
@@ -83,6 +93,8 @@ class FlutterTask extends DefaultTask {
     @OutputDirectory
     File intermediateDir
 
+    File engineSrcDir
+
     String getFlxPath() {
         return "${intermediateDir}/app.flx"
     }
@@ -97,6 +109,9 @@ class FlutterTask extends DefaultTask {
         project.exec {
             executable "${sdkDir}/bin/flutter"
             workingDir sourceDir
+            if (engineSrcDir != null) {
+              args "--engine-src-path", engineSrcDir
+            }
             args "build", "flx"
             args "-o", flxPath
             args "--snapshot", "${intermediateDir}/snapshot_blob.bin"