Use offset and count to request slices of lists, maps, and typed_data.

runtime/vm/json_stream.cc

Index: runtime/vm/json_stream.cc
diff --git a/runtime/vm/json_stream.cc b/runtime/vm/json_stream.cc
index 0f4f3a647dd79de3c8190b112486fa00e0f95663..0477bb48b78124206191ece696510a897d14ec91 100644
--- a/runtime/vm/json_stream.cc
+++ b/runtime/vm/json_stream.cc
@@ -30,7 +30,9 @@ JSONStream::JSONStream(intptr_t buf_size)
       method_(""),
       param_keys_(NULL),
       param_values_(NULL),
-      num_params_(0) {
+      num_params_(0),
+      offset_(0),
+      count_(-1) {
   ObjectIdRing* ring = NULL;
   Isolate* isolate = Isolate::Current();
   if (isolate != NULL) {
@@ -245,6 +247,22 @@ bool JSONStream::ParamIs(const char* key, const char* value) const {
 }
 
 
+void JSONStream::ComputeOffsetAndCount(intptr_t length,
+                                       intptr_t* offset,
+                                       intptr_t* count) {
+  // This function is written to avoid adding (count + offset) in case
+  // that triggers an integer overflow.
+  *offset = offset_;
+  if (*offset > length) {
+    *offset = length;
+  }
+  intptr_t remaining = length - *offset;
+  *count = count_;
+  if (*count < 0 || *count > remaining) {
+    *count = remaining;
+  }
+}
+
 void JSONStream::Clear() {
   buffer_.Clear();
   open_objects_ = 0;