Use the narrowest character width possible for substrings.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index 2c235afe6ac56b7269fdbb38a76a7712690428d9..7f6c3b2a18f64340402defe94bc8d8c06a7e7bbf 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -2431,10 +2431,10 @@ class OneByteString : public String {
                                      intptr_t len,
                                      Heap::Space space);
 
-  static RawOneByteString* SubString(const OneByteString& str,
-                                     intptr_t begin_index,
-                                     intptr_t length,
-                                     Heap::Space space);
+  static RawString* SubString(const OneByteString& str,
+                              intptr_t begin_index,
+                              intptr_t length,
+                              Heap::Space space);
 
  private:
   uint8_t* CharAddr(intptr_t index) const {
@@ -2482,10 +2482,10 @@ class TwoByteString : public String {
                                      intptr_t len,
                                      Heap::Space space);
 
-  static RawTwoByteString* SubString(const TwoByteString& str,
-                                     intptr_t begin_index,
-                                     intptr_t length,
-                                     Heap::Space space);
+  static RawString* SubString(const TwoByteString& str,
+                              intptr_t begin_index,
+                              intptr_t length,
+                              Heap::Space space);
 
  private:
   uint16_t* CharAddr(intptr_t index) const {
@@ -2529,10 +2529,10 @@ class FourByteString : public String {
                                       intptr_t len,
                                       Heap::Space space);
 
-  static RawFourByteString* SubString(const FourByteString& str,
-                                      intptr_t begin_index,
-                                      intptr_t length,
-                                      Heap::Space space);
+  static RawString* SubString(const FourByteString& str,
+                              intptr_t begin_index,
+                              intptr_t length,
+                              Heap::Space space);
 
  private:
   uint32_t* CharAddr(intptr_t index) const {