Make RegExp's constructor non-const.

sdk/lib/coreimpl/regexp.dart

Index: sdk/lib/coreimpl/regexp.dart
diff --git a/sdk/lib/coreimpl/regexp.dart b/sdk/lib/coreimpl/regexp.dart
index b7076311f31afeffbca66a6570cf538bb68d9552..845a26e38b83aa34dfdd35398e79003c3c727bb4 100644
--- a/sdk/lib/coreimpl/regexp.dart
+++ b/sdk/lib/coreimpl/regexp.dart
@@ -3,9 +3,9 @@
 // BSD-style license that can be found in the LICENSE file.
 
 class JSSyntaxRegExp implements RegExp {
-  external const JSSyntaxRegExp(String pattern,
-                                {bool multiLine: false,
-                                 bool ignoreCase: false});
+  external JSSyntaxRegExp(String pattern,
+                          {bool multiLine: false,
+                           bool ignoreCase: false});
   external Match firstMatch(String str);
   external Iterable<Match> allMatches(String str);
   external bool hasMatch(String str);