[mjsunit] Change assertThrows such that it can check the exception message.

test/mjsunit/mjsunit.js

Index: test/mjsunit/mjsunit.js
diff --git a/test/mjsunit/mjsunit.js b/test/mjsunit/mjsunit.js
index d4759a281f734c45072aad61d101da416bb03796..88d4d3677c4edc84fc2e9128737083cb246a64bb 100644
--- a/test/mjsunit/mjsunit.js
+++ b/test/mjsunit/mjsunit.js
@@ -376,7 +376,7 @@ var assertMatches;
         failWithMessage("invalid use of assertThrows, maybe you want assertThrowsEquals");
       }
       if (arguments.length >= 3) {
-        assertEquals(e.type, cause_opt);
+        assertEquals(e.message, cause_opt);
       }
       // Success.
       return;