Add some support for the code-point code-unit distinction.

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
diff --git a/runtime/vm/parser.cc b/runtime/vm/parser.cc
index 2ecd491588147630f1cddccc12e99cd718410310..4dd9f56745ed4bb179212fcb4af77f8c84163395 100644
--- a/runtime/vm/parser.cc
+++ b/runtime/vm/parser.cc
@@ -1378,7 +1378,7 @@ static RawClass* LookupImplClass(const String& class_name) {
 static RawClass* LookupCoreClass(const String& class_name) {
   const Library& core_lib = Library::Handle(Library::CoreLibrary());
   String& name = String::Handle(class_name.raw());
-  if (class_name.CharAt(0) == Scanner::kPrivateIdentifierStart) {
+  if (class_name.CodeUnitAt(0) == Scanner::kPrivateIdentifierStart) {
     // Private identifiers are mangled on a per script basis.
     name = String::Concat(name, String::Handle(core_lib.private_key()));
     name = Symbols::New(name);