VM: Speed up output of UTF8 for 1-byte strings.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index 343bc0bd4dc79bbfcf6b17c648543744852507d1..3c179214d5759a3f50f133b2745b98a553db905c 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -7166,6 +7166,11 @@ class OneByteString : public AllStatic {
     return &str.UnsafeMutableNonPointer(raw_ptr(str)->data())[index];
   }
 
+  static uint8_t* DataStart(const String& str) {
+    ASSERT(str.IsOneByteString());
+    return &str.UnsafeMutableNonPointer(raw_ptr(str)->data())[0];
+  }
+
   static RawOneByteString* ReadFrom(SnapshotReader* reader,
                                     intptr_t object_id,
                                     intptr_t tags,
@@ -7178,6 +7183,7 @@ class OneByteString : public AllStatic {
   friend class ExternalOneByteString;
   friend class SnapshotReader;
   friend class StringHasher;
+  friend class Utf8;
 };
 
 class TwoByteString : public AllStatic {
@@ -7283,6 +7289,11 @@ class TwoByteString : public AllStatic {
     return &str.UnsafeMutableNonPointer(raw_ptr(str)->data())[index];
   }
 
+  static uint16_t* DataStart(const String& str) {
+    ASSERT(str.IsTwoByteString());
+    return &str.UnsafeMutableNonPointer(raw_ptr(str)->data())[0];
+  }
+
   static RawTwoByteString* ReadFrom(SnapshotReader* reader,
                                     intptr_t object_id,
                                     intptr_t tags,
@@ -7349,6 +7360,11 @@ class ExternalOneByteString : public AllStatic {
     return &(raw_ptr(str)->external_data_->data()[index]);
   }
 
+  static const uint8_t* DataStart(const String& str) {
+    ASSERT(str.IsExternalOneByteString());
+    return &(raw_ptr(str)->external_data_->data()[0]);
+  }
+
   static void SetExternalData(const String& str,
                               ExternalStringData<uint8_t>* data) {
     ASSERT(str.IsExternalOneByteString());
@@ -7376,6 +7392,7 @@ class ExternalOneByteString : public AllStatic {
   friend class String;
   friend class SnapshotReader;
   friend class Symbols;
+  friend class Utf8;
 };
 
 class ExternalTwoByteString : public AllStatic {
@@ -7428,6 +7445,11 @@ class ExternalTwoByteString : public AllStatic {
     return &(raw_ptr(str)->external_data_->data()[index]);
   }
 
+  static const uint16_t* DataStart(const String& str) {
+    ASSERT(str.IsExternalTwoByteString());
+    return &(raw_ptr(str)->external_data_->data()[0]);
+  }
+
   static void SetExternalData(const String& str,
                               ExternalStringData<uint16_t>* data) {
     ASSERT(str.IsExternalTwoByteString());