Add newton Dart library to //third_party/dart-pkg

third_party/dart-pkg/BUILD.gn

Index: third_party/dart-pkg/BUILD.gn
diff --git a/third_party/dart-pkg/BUILD.gn b/third_party/dart-pkg/BUILD.gn
index 8df39ea5a1b12e21c39ade5fa72329c1e1d64de2..8f7c873b495fb8ad4a9e70dec38479ebe5e1f953 100644
--- a/third_party/dart-pkg/BUILD.gn
+++ b/third_party/dart-pkg/BUILD.gn
@@ -12,6 +12,7 @@ group("dart-pkg") {
     ":cassowary",
     ":collection",
     ":crypto",
+    ":newton",
     ":path",
     ":quiver",
     ":source_span",
@@ -45,6 +46,10 @@ dart_pkg("crypto") {
   pkg_dir = "//third_party/dart-pkg/crypto"
 }
 
+dart_pkg("newton") {
+  pkg_dir = "//third_party/dart-pkg/newton"
+}
+
 dart_pkg("path") {
   pkg_dir = "//third_party/dart-pkg/path"
 }