Make RegExp constructor non-const.

tests/compiler/dart2js/constant_folding_test.dart

Index: tests/compiler/dart2js/constant_folding_test.dart
diff --git a/tests/compiler/dart2js/constant_folding_test.dart b/tests/compiler/dart2js/constant_folding_test.dart
index e2133b7f70ee58e32d1347b2e43fb6510616dcd1..f803b25ec3dac0b3e699b9be8c711942fcbb267d 100644
--- a/tests/compiler/dart2js/constant_folding_test.dart
+++ b/tests/compiler/dart2js/constant_folding_test.dart
@@ -32,20 +32,20 @@ foo(a, b, c, d) {
 
 main() {
   compileAndMatch(
-      NUMBER_FOLDING, 'main', const RegExp(r"print\(7\)"));
+      NUMBER_FOLDING, 'main', new RegExp(r"print\(7\)"));
   compileAndMatch(
-      NEGATIVE_NUMBER_FOLDING, 'main', const RegExp(r"print\(1\)"));
+      NEGATIVE_NUMBER_FOLDING, 'main', new RegExp(r"print\(1\)"));
 
   String generated = compile(NULL_EQUALS_FOLDING, entry: 'foo');
-  RegExp regexp = const RegExp(r'a == null');
+  RegExp regexp = new RegExp(r'a == null');
   Expect.isTrue(regexp.hasMatch(generated));
 
-  regexp = const RegExp(r'null == b');
+  regexp = new RegExp(r'null == b');
   Expect.isTrue(regexp.hasMatch(generated));
 
-  regexp = const RegExp(r'4 === c');
+  regexp = new RegExp(r'4 === c');
   Expect.isTrue(regexp.hasMatch(generated));
 
-  regexp = const RegExp("'foo' === d");
+  regexp = new RegExp("'foo' === d");
   Expect.isTrue(regexp.hasMatch(generated));
 }