Added .message accessor to messages.

src/messages.js

 // Copyright 2006-2008 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 135 matching lines @@
     for (var i = 0; i < args.length; i++) {
       var elem = args[i];
       if (elem instanceof $Array && elem.length > 100) { // arbitrary limit, grab a reasonable slice to report
         args[i] = elem.slice(0,20).concat("...");
       }
     }
   } else if (IS_UNDEFINED(args)) {
     args = [];
   }
 
-  var e = new constructor();
+  var e = new constructor(kAddMessageAccessorsMarker);
   e.type = type;
   e.arguments = args;
   return e;
 }
 
 
 /**
  * Setup the Script function and constructor.
  */
 %FunctionSetInstanceClassName(Script, 'Script');
@@ skipping 415 matching lines @@
       if (!isTopLevel) result += ")";
     }
   }
   return (result) ? "    at " + result : result;
 }
 
 
 // ----------------------------------------------------------------------------
 // Error implementation
 
+// If this object gets passed to an error constructor the error will
+// get an accessor for .message that constructs a descriptive error
+// message on access.
+var kAddMessageAccessorsMarker = { };
+
+// Defines accessors for a property that is calculated the first time
+// the property is read and then replaces the accessor with the value.
+// Also, setting the property causes the accessors to be deleted.
+function DefineOneShotAccessor(obj, name, fun) {
+  // Note that the accessors consistently operate on 'obj', not 'this'.
+  // Since the object may occur in someone else's prototype chain we
+  // can't rely on 'this' being the same as 'obj'.
+  obj.__defineGetter__(name, function () {
+    var value = fun(obj);
+    obj[name] = value;
+    return value;
+  });
+  obj.__defineSetter__(name, function (v) {
+    delete obj[name];
+    obj[name] = v;
+  });
+}
+
 function DefineError(f) {
   // Store the error function in both the global object
   // and the runtime object. The function is fetched
   // from the runtime object when throwing errors from
   // within the runtime system to avoid strange side
   // effects when overwriting the error functions from
   // user code.
   var name = f.name;
   %SetProperty(global, name, f, DONT_ENUM);
   this['$' + name] = f;
   // Configure the error function.
   // prototype of 'Error' must be as default: new Object().
   if (name != 'Error') %FunctionSetPrototype(f, new $Error());
   %FunctionSetInstanceClassName(f, 'Error');
   %SetProperty(f.prototype, 'constructor', f, DONT_ENUM);
   f.prototype.name = name;
   %SetCode(f, function(m) {
     if (%IsConstructCall()) {
-      if (!IS_UNDEFINED(m)) this.message = ToString(m);
+      if (m === kAddMessageAccessorsMarker) {
+        DefineOneShotAccessor(this, 'message', function (obj) {
+          return FormatMessage({type: obj.type, args: obj.arguments});
+        });
+      } else if (!IS_UNDEFINED(m)) {
+        this.message = ToString(m);
+      }
     } else {
       return new f(m);
     }
   });
 }
 
 $Math.__proto__ = global.Object.prototype;
 
 DefineError(function Error() { });
 DefineError(function TypeError() { });
@@ skipping 12 matching lines @@
     return this.name + ": " + FormatMessage({ type: type, args: this.arguments });
   }
   var message = this.message;
   return this.name + (message ? (": " + message) : "");
 }, DONT_ENUM);
 
 
 // Boilerplate for exceptions for stack overflows. Used from
 // Top::StackOverflow().
 const kStackOverflowBoilerplate = MakeRangeError('stack_overflow', []);