mjsunit: Fix the error message produced by assertInstanceof.

Used to be:

Failure: expected <foo> is not an instance of <Bar> but of < Baz>> found <undefined>

Should be:

Failure: <foo> is not an instance of <Bar> but of <Baz>

BUG=

Review-Url: https://codereview.chromium.org/2413153004
Cr-Commit-Position: refs/heads/master@{#40319}

test/mjsunit/mjsunit.js

@@ -206,13 +206,16 @@ var assertMatches;
 
   function fail(expectedText, found, name_opt) {
     var message = "Fail" + "ure";
-    if (name_opt) {
-      // Fix this when we ditch the old test runner.
-      message += " (" + name_opt + ")";
+    if (found) {
+      message += ": expected <" + expectedText +
+          "> found <" + PrettyPrint(found) + ">";
+      if (name_opt) {
+        // Fix this when we ditch the old test runner.
+        message += " (" + name_opt + ")";
+      }
+    } else {
+      message += ": " + expectedText;
     }
-
-    message += ": expected <" + expectedText +
-        "> found <" + PrettyPrint(found) + ">";
     throw new MjsUnitAssertionError(message);
   }
 
@@ -396,7 +399,7 @@ var assertMatches;
       }
       fail("Object <" + PrettyPrint(obj) + "> is not an instance of <" +
                (type.name || type) + ">" +
-               (actualTypeName ? " but of < " + actualTypeName + ">" : ""));
+               (actualTypeName ? " but of <" + actualTypeName + ">" : ""));
     }
   };