cps-ir: Don't bail on assert in unchecked mode.

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 '../constants/expressions.dart';
 import '../dart_types.dart';
@@ skipping 618 matching lines @@
     // Throw away the result of visiting the expression.
     // Instead we return the result of visiting the CascadeReceiver.
     this.visit(node.expression);
     ir.Primitive receiver = _currentCascadeReceiver;
     _currentCascadeReceiver = oldCascadeReceiver;
     return receiver;
   }
 
   // ## Sends ##
   @override
-  ir.Primitive visitAssert(
-      ast.Send node,
-      ast.Node condition,
-      _) {
+  ir.Primitive visitAssert(ast.Send node, ast.Node condition, _) {
     assert(irBuilder.isOpen);
-    return giveup(node, 'Assert');
+    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;
+    }
   }
 
   ir.Primitive visitNamedArgument(ast.NamedArgument node) {
     assert(irBuilder.isOpen);
     return visit(node.expression);
   }
 
   @override
   ir.Primitive visitExpressionInvoke(ast.Send node,
                                      ast.Node expression,
@@ skipping 2082 matching lines @@
   SourceInformation buildCall(ast.Node node) {
     return new PositionSourceInformation(
         new TokenSourceLocation(sourceFile, node.getBeginToken(), name));
   }
 
   @override
   SourceInformationBuilder forContext(AstElement element) {
     return new PositionSourceInformationBuilder(element);
   }
 }