Add option to disable MirrorCache.

test/mjsunit/debug-toggle-mirror-cache.js

Index: test/mjsunit/debug-toggle-mirror-cache.js
diff --git a/test/mjsunit/es6/regress/regress-cr372788.js b/test/mjsunit/debug-toggle-mirror-cache.js
similarity index 78%
copy from test/mjsunit/es6/regress/regress-cr372788.js
copy to test/mjsunit/debug-toggle-mirror-cache.js
index 9b66a7e08b3737cf26b0dc52e87f4e69bddbf729..a44c11551e02125fe8cd54b693c5ac6dee545721 100644
--- a/test/mjsunit/es6/regress/regress-cr372788.js
+++ b/test/mjsunit/debug-toggle-mirror-cache.js
@@ -25,21 +25,16 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-// Flags: --allow-natives-syntax
+// Flags: --expose-debug-as debug
 
-var x = 0;
-var y = 0;
+var handle1 = debug.MakeMirror(123).handle();
+assertEquals("number", debug.LookupMirror(handle1).type());
 
-var thenable = { then: function(f) { x++; f(); } };
+debug.ToggleMirrorCache(false);
+var handle2 = debug.MakeMirror(123).handle();
+assertEquals(undefined, handle2);
+assertThrows(function() { debug.LookupMirror(handle2) });
 
-for (var i = 0; i < 3; ++i) {
-  Promise.resolve(thenable).then(function() { x++; y++; });
-}
-assertEquals(0, x);
-
-(function check() {
-  Promise.resolve().chain(function() {
-    // Delay check until all handlers have run.
-    if (y < 3) check(); else assertEquals(6, x);
-  }).catch(function(e) { %AbortJS("FAILURE: " + e) });
-})();
+debug.ToggleMirrorCache(true);
+var handle3 = debug.MakeMirror(123).handle();
+assertEquals("number", debug.LookupMirror(handle3).type());