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

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
diff --git a/runtime/vm/dart_api_impl.cc b/runtime/vm/dart_api_impl.cc
index 202a81fd5bdc560a189954585a4f80466eae9376..6a5a9bf82c68ca698cc2b659b0347ee59a30e122 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -1700,7 +1700,7 @@ DART_EXPORT Dart_Handle Dart_StringToUTF16(Dart_Handle str,
   intptr_t str_len = str_obj.Length();
   intptr_t copy_len = (str_len > *length) ? *length : str_len;
   for (intptr_t i = 0; i < copy_len; i++) {
-    utf16_array[i] = static_cast<uint16_t>(str_obj.CharAt(i));
+    utf16_array[i] = static_cast<uint16_t>(str_obj.CodeUnitAt(i));
   }
   *length = copy_len;
   return Api::Success(isolate);
@@ -2598,7 +2598,7 @@ DART_EXPORT Dart_Handle Dart_ClassGetFunctionTypeSignature(Dart_Handle clazz) {
 
 // Outside of the vm, we expose setter names with a trailing '='.
 static bool HasExternalSetterSuffix(const String& name) {
-  return name.CharAt(name.Length() - 1) == '=';
+  return name.CodeUnitAt(name.Length() - 1) == '=';
 }