VM: Optimized code for all of [External]{One|Two}ByteString::codeUnitAt.

runtime/lib/string_patch.dart

Index: runtime/lib/string_patch.dart
diff --git a/runtime/lib/string_patch.dart b/runtime/lib/string_patch.dart
index f7aa83c5522b3d31143e1f73e4e3ecb0225d9f29..435224516636f1bc51b04bcda0824ede8c8b8b2f 100644
--- a/runtime/lib/string_patch.dart
+++ b/runtime/lib/string_patch.dart
@@ -230,7 +230,7 @@ abstract class _StringBase {
 
   String operator [](int index) native "String_charAt";
 
-  int codeUnitAt(int index) native "String_codeUnitAt";
+  int codeUnitAt(int index);  // Implemented in the subclasses.
 
   int get length native "String_getLength";
 
@@ -937,6 +937,8 @@ class _OneByteString extends _StringBase implements String {
 
   int get hashCode native "String_getHashCode";
 
+  int codeUnitAt(int index) native "String_codeUnitAt";
+
   bool _isWhitespace(int codeUnit) {
     return _StringBase._isOneByteWhitespace(codeUnit);
   }
@@ -1240,6 +1242,8 @@ class _TwoByteString extends _StringBase implements String {
     return _StringBase._isTwoByteWhitespace(codeUnit);
   }
 
+  int codeUnitAt(int index) native "String_codeUnitAt";
+
   bool operator ==(Object other) {
     return super == other;
   }
@@ -1256,6 +1260,8 @@ class _ExternalOneByteString extends _StringBase implements String {
     return _StringBase._isOneByteWhitespace(codeUnit);
   }
 
+  int codeUnitAt(int index) native "String_codeUnitAt";
+
   bool operator ==(Object other) {
     return super == other;
   }
@@ -1274,6 +1280,8 @@ class _ExternalTwoByteString extends _StringBase implements String {
     return _StringBase._isTwoByteWhitespace(codeUnit);
   }
 
+  int codeUnitAt(int index) native "String_codeUnitAt";
+
   bool operator ==(Object other) {
     return super == other;
   }