Introduce per-isolate cache for compile time constants

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index f59846deb74321e36777d443c89634c572baf535..e2b616e06fda73c1d306125ebc4feb48c57ff849 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -18130,6 +18130,11 @@ static intptr_t HashImpl(const T* characters, intptr_t len) {
 }
 
 
+intptr_t String::Hash(const char* characters, intptr_t len) {
+  return HashImpl(characters, len);
+}
+
+
 intptr_t String::Hash(const uint8_t* characters, intptr_t len) {
   return HashImpl(characters, len);
 }