Update references to the engine project. (#12515)

examples/flutter_gallery/BUILD.gn

@@ -2,7 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//flutter/build/flutter_app.gni")
+import("$flutter_root/build/flutter_app.gni")
 
 assert(is_fuchsia)
 

packages/flutter/BUILD.gn

@@ -20,6 +20,6 @@ dart_package("flutter") {
   ]
 
   if (is_fuchsia) {
-    deps += [ "//flutter/sky/packages/sky_engine:sky_engine_dart" ]
+    deps += [ "$flutter_root/sky/packages/sky_engine:sky_engine_dart" ]
   }
 }

packages/flutter_tools/BUILD.gn

@@ -59,6 +59,6 @@ dart_tool("fuchsia_tester") {
   ]
 
   non_dart_deps = [
-    "//flutter/shell",
+    "$flutter_root/shell",
   ]
 }