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

tools/testing/dart/environment.dart

 // Copyright (c) 2017, 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 'configuration.dart';
 
 typedef String _LookUpFunction(Configuration configuration);
 typedef bool _BoolLookUpFunction(Configuration configuration);
 
 // TODO(29756): Instead of synthesized negated variables like "unchecked",
 // consider adding support for "!" to status expressions.
 final _variables = {
   "analyzer": new _Variable.bool((c) => c.compiler == Compiler.dart2analyzer),
   "arch": new _Variable((c) => c.architecture.name, Architecture.names),
   "browser": new _Variable.bool((c) => c.runtime.isBrowser),
   "builder_tag": new _Variable((c) => c.builderTag ?? "", const []),
   "checked": new _Variable.bool((c) => c.isChecked),
   "compiler": new _Variable((c) => c.compiler.name, Compiler.names),
   "csp": new _Variable.bool((c) => c.isCsp),
   "dart2js_with_kernel": new _Variable.bool((c) => c.useDart2JSWithKernel),
   "fast_startup": new _Variable.bool((c) => c.useFastStartup),
+  "enable_asserts": new _Variable.bool((c) => c.useEnableAsserts),
   "host_checked": new _Variable.bool((c) => c.isHostChecked),
   "host_unchecked": new _Variable.bool((c) => !c.isHostChecked),
   "hot_reload": new _Variable.bool((c) => c.hotReload),
   "hot_reload_rollback": new _Variable.bool((c) => c.hotReloadRollback),
   "ie": new _Variable.bool((c) => c.runtime.isIE),
   "jscl": new _Variable.bool((c) => c.runtime.isJSCommandLine),
   "minified": new _Variable.bool((c) => c.isMinified),
   "mode": new _Variable((c) => c.mode.name, Mode.names),
   "runtime": new _Variable(_runtimeName, Runtime.names),
   "strong": new _Variable.bool((c) => c.isStrong),
@@ skipping 72 matching lines @@
   _Variable(this._lookUp, Iterable<String> allowed)
       : allowedValues = allowed.toList();
 
   /// Creates a Boolean variable with allowed values "true" and "false".
   _Variable.bool(_BoolLookUpFunction lookUp)
       : _lookUp = ((configuration) => lookUp(configuration).toString()),
         allowedValues = const ["true", "false"];
 
   String lookUp(Configuration configuration) => _lookUp(configuration);
 }