Add optional message to assert in Dart2js.

pkg/compiler/lib/src/cps_ir/cps_ir_builder_task.dart

 // Copyright (c) 2013, 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.
 
 library dart2js.ir_builder_task;
 
 import '../closure.dart' as closurelib;
 import '../closure.dart' hide ClosureScope;
 import '../common/names.dart' show
     Names,
@@ skipping 632 matching lines @@
     // Throw away the result of visiting the expression.
     // Instead we return the result of visiting the CascadeReceiver.
     visit(node.expression);
     ir.Primitive receiver = _currentCascadeReceiver;
     _currentCascadeReceiver = oldCascadeReceiver;
     return receiver;
   }
 
   // ## Sends ##
   @override
-  ir.Primitive visitAssert(ast.Send node, ast.Node condition, _) {
-    assert(irBuilder.isOpen);
-    if (compiler.enableUserAssertions) {
-      return giveup(node, 'assert in checked mode not implemented');
-    } else {
-      // The call to assert and its argument expression must be ignored
-      // in production mode.
-      // Assertions can only occur in expression statements, so no value needs
-      // to be returned.
-      return null;
-    }
-  }
-
-  @override
   void previsitDeferredAccess(ast.Send node, PrefixElement prefix, _) {
     giveup(node, 'deferred access is not implemented');
   }
 
   ir.Primitive visitNamedArgument(ast.NamedArgument node) {
     assert(irBuilder.isOpen);
     return visit(node.expression);
   }
 
   @override
@@ skipping 2893 matching lines @@
     if (compiler.backend.isForeign(function)) {
       return handleForeignCode(node, function, argumentList, callStructure);
     } else {
       return irBuilder.buildStaticFunctionInvocation(function, callStructure,
           translateStaticArguments(argumentList, function, callStructure),
           sourceInformation:
               sourceInformationBuilder.buildCall(node, node.selector));
     }
   }
 }