Add support to dart2js for option --enable-asserts.

tests/language/function_type/test_generator.dart

 // Copyright (c) 2016, 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.
 
 import 'dart:io';
 
 // By convention:
 //
 //   T: generic type of typedef.
 //   A: generic type of returned function.
@@ skipping 523 matching lines @@
 // GENERATED - DON'T EDIT.
 // GENERATED - DON'T EDIT.
 
 import 'dart:core';
 import 'dart:core' as core;
 import 'package:expect/expect.dart';
 
 @NoInline()
 @AssumeDynamic()
 confuse(f) => f;
-
-final bool inCheckedMode =
-    (() { bool result = false; assert(result = true); return result; })();
 """;
 
 class Unit {
   int typeCounter = 0;
   final String name;
   final StringBuffer typedefs = new StringBuffer();
   final StringBuffer globals = new StringBuffer();
   final StringBuffer tests = new StringBuffer();
   final StringBuffer fields = new StringBuffer();
   final StringBuffer statics = new StringBuffer();
@@ skipping 78 matching lines @@
 //   }
 final TYPEDEF_T_TESTS_TEMPLATE = """
     if (!tIsBool) {
       Expect.isTrue(#staticFunName is #typeName<int>);
       Expect.isFalse(#staticFunName is #typeName<bool>);
       Expect.isTrue(confuse(#staticFunName) is #typeName<int>);
       Expect.isFalse(confuse(#staticFunName) is #typeName<bool>);
       Expect.equals(tIsDynamic, #methodFunName is #typeName<bool>);
       Expect.equals(tIsDynamic, confuse(#methodFunName) is #typeName<bool>);
     } else {
-      if (inCheckedMode) {
+      if (typeAssertionsEnabled) {
         Expect.throws(() { #fieldName = (#staticFunName as dynamic); });
         Expect.throws(() { #fieldName = confuse(#staticFunName); });
         #typeCode #localName;
         Expect.throws(() { #localName = (#staticFunName as dynamic); });
         Expect.throws(() { #localName = confuse(#staticFunName); });
       }
       #typeCode #localName = #methodFunName;
       // In checked mode, verifies the type.
       #fieldName = #methodFunName;
       #fieldName = confuse(#methodFunName);
@@ skipping 104 matching lines @@
 """);
 }
 
 void main(List<String> arguments) {
   if (arguments.length != 0) {
     printUsage();
     return;
   }
   generateTests();
 }