Add CodeStubAssembler::FlattenString

src/code-stub-assembler.h

Index: src/code-stub-assembler.h
diff --git a/src/code-stub-assembler.h b/src/code-stub-assembler.h
index 919df75fc2bfe5f75e86aec8119e58dd0a15def0..ae05a6c5294a26a39cf65f5b35fb57712209c0bd 100644
--- a/src/code-stub-assembler.h
+++ b/src/code-stub-assembler.h
@@ -644,6 +644,9 @@ class V8_EXPORT_PRIVATE CodeStubAssembler : public compiler::CodeAssembler {
   compiler::Node* JSReceiverToPrimitive(compiler::Node* context,
                                         compiler::Node* input);
 
+  // Convert a String to a flat String.
+  compiler::Node* FlattenString(compiler::Node* string);
+
   enum ToIntegerTruncationMode {
     kNoTruncation,
     kTruncateMinusZero,