[sdk] add gen_snapshot dart prebuilts in the sdk

They are needed to build AOT dart binaries using the SDK.

Bug: DX-1151
Change-Id: I0b9e7f00fd84922c7abfa8de2514d34b6752bf53
diff --git a/runtime/dart/BUILD.gn b/runtime/dart/BUILD.gn
index 1362c99..dbd3676 100644
--- a/runtime/dart/BUILD.gn
+++ b/runtime/dart/BUILD.gn
@@ -76,6 +76,14 @@
       dest = "tools/dart"
     },
     {
+      source = prebuilt_gen_snapshot
+      dest = "tools/dart_prebuilts/gen_snapshot.snapshot"
+    },
+    {
+      source = prebuilt_gen_snapshot_product
+      dest = "tools/dart_prebuilts/gen_snapshot_product.snapshot"
+    },
+    {
       source = "$root_out_dir/dart_runner_patched_sdk/platform_strong.dill"
       dest = "tools/dart_prebuilts/dart_runner/platform_strong.dill"
     },