Flatten cons string for single character substrings.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index f28c9260b9d8ebc211381ed971db3bbfaad88977..cf9d6c709ee489f0658196d3244c96c12e822c1c 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -3968,7 +3968,12 @@ MaybeObject* Heap::AllocateSubString(String* buffer,
   int length = end - start;
   if (length <= 0) {
     return empty_string();
-  } else if (length == 1) {
+  }
+
+  // Make an attempt to flatten the buffer to reduce access time.
+  buffer = buffer->TryFlattenGetString();
+
+  if (length == 1) {
     return LookupSingleCharacterStringFromCode(buffer->Get(start));
   } else if (length == 2) {
     // Optimization for 2-byte strings often used as keys in a decompression
@@ -3979,9 +3984,6 @@ MaybeObject* Heap::AllocateSubString(String* buffer,
     return MakeOrFindTwoCharacterString(this, c1, c2);
   }
 
-  // Make an attempt to flatten the buffer to reduce access time.
-  buffer = buffer->TryFlattenGetString();
-
   if (!FLAG_string_slices ||
       !buffer->IsFlat() ||
       length < SlicedString::kMinLength ||
@@ -4981,7 +4983,7 @@ MaybeObject* Heap::ReinitializeJSGlobalProxy(JSFunction* constructor,
 
 
 MaybeObject* Heap::AllocateStringFromOneByte(Vector<const uint8_t> string,
-                                           PretenureFlag pretenure) {
+                                             PretenureFlag pretenure) {
   int length = string.length();
   if (length == 1) {
     return Heap::LookupSingleCharacterStringFromCode(string[0]);