Enable map transitions for accessor properties.

src/regexp.js

Index: src/regexp.js
diff --git a/src/regexp.js b/src/regexp.js
index 44f8dd1234d9055afecd6be061839725b65f7d7d..38090397aa21c037b6f2b047d947c026d7b3cf02 100644
--- a/src/regexp.js
+++ b/src/regexp.js
@@ -427,6 +427,7 @@ function SetUpRegExp() {
     LAST_INPUT(lastMatchInfo) = ToString(string);
   };
 
+  %OptimizeObjectForAddingMultipleProperties($RegExp, 22);
   %DefineOrRedefineAccessorProperty($RegExp, 'input', RegExpGetInput,
                                     RegExpSetInput, DONT_DELETE);
   %DefineOrRedefineAccessorProperty($RegExp, '$_', RegExpGetInput,
@@ -481,6 +482,7 @@ function SetUpRegExp() {
                                       RegExpMakeCaptureGetter(i), NoOpSetter,
                                       DONT_DELETE);
   }
+  %ToFastProperties($RegExp);
 }
 
 SetUpRegExp();