Change VM's string-buffer patch to use a Uin16Array as backing buffer.

runtime/lib/string.cc

Index: runtime/lib/string.cc
diff --git a/runtime/lib/string.cc b/runtime/lib/string.cc
index 3b5b633a9e9802114463d9bfd90b71426f257511..5b27b116997d49b8a4e66ba153543fd37136758f 100644
--- a/runtime/lib/string.cc
+++ b/runtime/lib/string.cc
@@ -49,7 +49,6 @@ DEFINE_NATIVE_ENTRY(StringBase_createFromCodePoints, 1) {
   return TwoByteString::New(utf16_len, utf32_array, array_len, Heap::kNew);
 }
 
-
 DEFINE_NATIVE_ENTRY(StringBase_substringUnchecked, 3) {
   const String& receiver = String::CheckedHandle(arguments->NativeArgAt(0));
   GET_NON_NULL_NATIVE_ARGUMENT(Smi, start_obj, arguments->NativeArgAt(1));
@@ -197,4 +196,22 @@ DEFINE_NATIVE_ENTRY(Strings_concatAll, 1) {
   return String::ConcatAll(strings);
 }
 
+DEFINE_NATIVE_ENTRY(StringBuffer_createStringFromUint16Array, 2) {
+  GET_NON_NULL_NATIVE_ARGUMENT(Uint16Array, a, arguments->NativeArgAt(0));
+  GET_NON_NULL_NATIVE_ARGUMENT(Smi, smiLength, arguments->NativeArgAt(1));
+  intptr_t array_len = a.Length();
+  int32_t length = smiLength.Value();
+  if (length < 0 || length > array_len) {
+    const Array& args = Array::Handle(Array::New(1));
+    args.SetAt(0, smiLength);
+    Exceptions::ThrowByType(Exceptions::kRange, args);
+  }
+  // TODO(lrn): Pass parameter saying whether the data is Latin1, and
+  // use RawOneByteString::New/RawTwoByteString::New directly.
+  uint16_t* data_position = reinterpret_cast<uint16_t*>(
+      reinterpret_cast<char*>(a.raw()) +
+      Uint16Array::data_offset() - kHeapObjectTag);
+  return String::FromUTF16(data_position, length, Heap::kNew);

[Vyacheslav Egorov (Google), 2013/03/05 14:44:32]

This is not GC safe and is against VM guidelines. 

String::FromUTF16 first allocates new string and then copies content into it. 

Which means your data might get moved to some other place under your feet, if GC
happens when new string is allocated and you will end up copying garbage.

In VM we only pass handles into functions (with a few exceptions), raw pointers
into the middle are not allowed.

+}
+
 }  // namespace dart