Add fast path to ListGetAsBytes for byte-size ExternalTypedData.

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
===================================================================
--- runtime/vm/dart_api_impl.cc	(revision 36670)
+++ runtime/vm/dart_api_impl.cc	(working copy)
@@ -2686,12 +2686,11 @@
   }                                                                            \
   return Api::NewError("Invalid length passed in to access array elements");   \
 
-
-static Dart_Handle CopyBytes(const TypedData& array,
+template<typename T>
+static Dart_Handle CopyBytes(const T& array,
                              intptr_t offset,
                              uint8_t* native_array,
                              intptr_t length) {
-  ASSERT(array.IsTypedData());
   ASSERT(array.ElementSizeInBytes() == 1);
   NoGCScope no_gc;
   memmove(native_array,
@@ -2718,6 +2717,16 @@
       return CopyBytes(array, offset, native_array, length);
     }
   }
+  if (obj.IsExternalTypedData()) {
+    const ExternalTypedData& external_array = ExternalTypedData::Cast(obj);
+    if (external_array.ElementSizeInBytes() == 1) {
+      if (!Utils::RangeCheck(offset, length, external_array.Length())) {
+        return Api::NewError(
+            "Invalid length passed in to access list elements");
+      }
+      return CopyBytes(external_array, offset, native_array, length);
+    }
+  }
   if (RawObject::IsTypedDataViewClassId(obj.GetClassId())) {
     const Instance& view = Instance::Cast(obj);
     if (TypedDataView::ElementSizeInBytes(view) == 1) {