Add Fuchsia package() declaration (flutter#4239)

This declaration makes it easier for Fuchsia to include Flutter in the system
image.

BUILD.gn

@@ -5,7 +5,7 @@
 group("flutter") {
   testonly = true
 
-  deps = [
+  public_deps = [
     "$flutter_root/lib/snapshot:compile_platform",
     "$flutter_root/lib/snapshot:generate_snapshot_bin",
     "$flutter_root/sky",
@@ -14,15 +14,15 @@ group("flutter") {
 
   if (!is_fuchsia) {
     if (current_toolchain == host_toolchain) {
-      deps += [
-        "//dart:create_sdk",
+      public_deps += [
         "$flutter_root/frontend_server",
+        "//dart:create_sdk",
       ]
     }
   }
 
   if (is_fuchsia) {
-    deps += [
+    public_deps += [
       "$flutter_root/content_handler",
       "$flutter_root/content_handler:aot_content_handler",
       "$flutter_root/examples",
@@ -33,12 +33,13 @@ group("flutter") {
   # If on the host, compile all unittests targets.
   if (current_toolchain == host_toolchain) {
     if (is_mac) {
-      deps += [ "$flutter_root/shell/platform/darwin:flutter_channels_unittests" ]
+      public_deps +=
+          [ "$flutter_root/shell/platform/darwin:flutter_channels_unittests" ]
     }
     if (!is_win) {
-      deps += [ "$flutter_root/shell/platform/embedder:flutter_engine" ]
+      public_deps += [ "$flutter_root/shell/platform/embedder:flutter_engine" ]
     }
-    deps += [
+    public_deps += [
       "$flutter_root/flow:flow_unittests",
       "$flutter_root/fml:fml_unittests",
       "$flutter_root/sky/engine/wtf:wtf_unittests",
@@ -51,12 +52,30 @@ group("flutter") {
 }
 
 config("config") {
-  include_dirs = [
-    "..",
-  ]
+  include_dirs = [ ".." ]
 }
 
-if (!is_fuchsia) {
+if (is_fuchsia) {
+  import("//packages/package.gni")
+
+  package("package") {
+    app = true
+    testonly = true
+
+    package_name = "flutter"
+
+    deps = [ ":flutter" ]
+
+    binaries = [
+      {
+        name = "flutter_runner"
+      },
+      {
+        name = "flutter_aot_runner"
+      },
+    ]
+  }
+} else {
   group("dist") {
     testonly = true