Adds typechecking of return values

frog/corejs.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Generates JS helpers for dart:core. This used to be in a file "core.js".
  * Having them in Dart code means we can easily control which are generated.
  */
 // TODO(jmesserly): one idea to make this cleaner: put these as private "native"
 // methods somewhere in a library that we import. This would be rather elegant
@@ skipping 269 matching lines @@
   // TODO(jmesserly): setting the stack property is not a long term solution.
   // Also it causes the exception to print as if it were a TypeError or
   // RangeError, instead of using the proper toString.
   res.stack = e.stack;
   return res;
 }""");
     }
 
     if (useNotNullBool) {
       useThrow = true;
+      // Some testing showed that this patterned fared well across browsers.
       w.writeln(@"""
 function $notnull_bool(test) {
-  if (test == null || typeof(test) != 'boolean') {
-    $throw(new TypeError('must be "true" or "false"'));
-  }
-  return test === true;
+  return typeof(test) == 'boolean' ? test : test.is$bool();
 }""");
     }
 
     if (useAssert) {
       useThrow = true;
       w.writeln(@"""
 function $assert(test, text, url, line, column) {
   if (typeof test == 'function') test = test();
   if (!test) $throw(new AssertError(text, url, line, column));
 }""");
@@ skipping 78 matching lines @@
 Object.prototype.$setindex = function(i, value) { return this[i] = value; }
 Array.prototype.$setindex = function(i, value) { return this[i] = value; }""");
     }
 
     // Write operator helpers
     for (var opImpl in orderValuesByKeys(_usedOperators)) {
       w.writeln(opImpl);
     }
   }
 }