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

test/mjsunit/regress/regress-2263.js

 // Copyright 2012 the V8 project authors. All rights reserved.
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 // met:
 //
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 //       notice, this list of conditions and the following disclaimer.
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+assertThrows = function assertThrows(code, type_opt, cause_opt) {
+  var threwException = true;
+  try {
+    if (typeof code === 'function') {
+      code();
+    } else {
+      eval(code);
+    }
+    threwException = false;
+  } catch (e) {
+    if (typeof type_opt === 'function') {
+      assertInstanceof(e, type_opt);
+    } else if (type_opt !== void 0) {
+      failWithMessage("invalid use of assertThrows, maybe you want assertThrowsEquals");
+    }
+    if (arguments.length >= 3) {
+      assertEquals(e.type, cause_opt);
+    }
+    // Success.
+    return;
+  }
+  failWithMessage("Did not throw exception");
+};
+
 var obj = { length: { valueOf: function(){ throw { type: "length" }}}};
 var sep = { toString: function(){ throw { type: "toString" }}};
 assertThrows("Array.prototype.join.call(obj, sep)", undefined, "length");