Stop running pub get at gclient sync time and fix build bugs

mojo/dart/dart_snapshotter/BUILD.gn

Index: mojo/dart/dart_snapshotter/BUILD.gn
diff --git a/mojo/dart/dart_snapshotter/BUILD.gn b/mojo/dart/dart_snapshotter/BUILD.gn
index 4174569a113ea5100e039147111b8fba00463af1..f46f491d95b2472b150c1fa07b876e4e100b126f 100644
--- a/mojo/dart/dart_snapshotter/BUILD.gn
+++ b/mojo/dart/dart_snapshotter/BUILD.gn
@@ -12,8 +12,6 @@ executable("dart_snapshotter") {
   deps = [
     "//base",
     "//build/config/sanitizers:deps",
-    "//dart/runtime/vm:libdart_platform",
-    "//dart/runtime:libdart",
     "//mojo/common",
     "//mojo/dart/embedder:dart_snapshot_cc",
     "//mojo/edk/system",
@@ -21,6 +19,8 @@ executable("dart_snapshotter") {
     "//mojo/public/cpp/system",
     "//mojo/public/cpp/utility",
     "//mojo/public/interfaces/application",
+    "//mojo/public/third_party/dart/runtime/vm:libdart_platform",
+    "//mojo/public/third_party/dart/runtime:libdart",
     "//third_party/zlib",
     "//tonic",
   ]