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 9d5b4d9e2eaafd9542d20e3c6b44d3414870b892..1796f940f6e1184a653cd711b1e19cc0a7dba1fd 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -7244,7 +7244,7 @@ class OneByteString : public AllStatic {
   }
 
   static uint8_t* CharAddr(const String& str, intptr_t index) {
-    ASSERT((index >= 0) && (index < str.Length()));
+    ASSERT(index == 0 || ((index >= 0) && (index < str.Length())));
     ASSERT(str.IsOneByteString());
     return &str.UnsafeMutableNonPointer(raw_ptr(str)->data())[index];
   }
@@ -7261,6 +7261,7 @@ class OneByteString : public AllStatic {
   friend class ExternalOneByteString;
   friend class SnapshotReader;
   friend class StringHasher;
+  friend class Utf8;
 };
 
 
@@ -7430,7 +7431,7 @@ class ExternalOneByteString : public AllStatic {
   }
 
   static const uint8_t* CharAddr(const String& str, intptr_t index) {
-    ASSERT((index >= 0) && (index < str.Length()));
+    ASSERT(index == 0 || ((index >= 0) && (index < str.Length())));
     ASSERT(str.IsExternalOneByteString());
     return &(raw_ptr(str)->external_data_->data()[index]);
   }
@@ -7462,6 +7463,7 @@ class ExternalOneByteString : public AllStatic {
   friend class String;
   friend class SnapshotReader;
   friend class Symbols;
+  friend class Utf8;
 };