Move JSSyntaxRegExp to core as a private member. This removes the last refrences to dart:coreimpl.

runtime/vm/snapshot_test.cc

Index: runtime/vm/snapshot_test.cc
diff --git a/runtime/vm/snapshot_test.cc b/runtime/vm/snapshot_test.cc
index d0e57def9851c648dc2ae602cda810289685717c..3ee8ed704add759f6eee6a22daa2b842f43d7341 100644
--- a/runtime/vm/snapshot_test.cc
+++ b/runtime/vm/snapshot_test.cc
@@ -867,11 +867,6 @@ TEST_CASE(GenerateSource) {
   EXPECT(!lib.IsNull());
   IterateScripts(lib);
 
-  // Check core impl lib.
-  lib = Library::CoreImplLibrary();
-  EXPECT(!lib.IsNull());
-  IterateScripts(lib);
-
   // Check isolate lib.
   lib = Library::IsolateLibrary();
   EXPECT(!lib.IsNull());