Fix source property of empty RegExp objects.

test/mjsunit/regress/regress-1217.js

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 12 matching lines @@
 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 // Check that RegExp.prototype is itself a RegExp object.
 
 var proto = RegExp.prototype;
 assertEquals("[object RegExp]", Object.prototype.toString.call(proto));
 
-assertEquals("", proto.source);
+assertEquals("(?:)", proto.source);
 assertEquals(false, proto.global);
 assertEquals(false, proto.multiline);
 assertEquals(false, proto.ignoreCase);
 assertEquals(0, proto.lastIndex);
 
 assertEquals("/(?:)/", proto.toString());
 
 var execResult = proto.exec("argle");
 assertEquals(1, execResult.length);
 assertEquals("", execResult[0]);
 assertEquals("argle", execResult.input);
 assertEquals(0, execResult.index);
 
 assertTrue(proto.test("argle"));
 
 // We disallow re-compiling the RegExp.prototype object.
 assertThrows(function(){ proto.compile("something"); }, TypeError);