Check boolean conversion in checked mode.

lib/compiler/implementation/js_backend/emitter.dart

 // Copyright (c) 2012, 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.
 
 /**
  * A function element that represents a closure call. The signature is copied
  * from the given element.
  */
 class ClosureInvocationElement extends FunctionElement {
   ClosureInvocationElement(SourceString name,
@@ skipping 1200 matching lines @@
       mainEnsureGetter = "$mainAccess.$invocationName = $mainAccess";
     }
 
     // TODO(ngeoffray): These globals are currently required by the isolate
     // library, but since leg already generates code on an Isolate object, they
     // are not really needed. We should remove them once Leg replaces Frog.
     buffer.add("""
 var \$globalThis = $currentIsolate;
 var \$globalState;
 var \$globals;
-var \$isWorker;
-var \$supportsWorkers;
+var \$isWorker = false;
+var \$supportsWorkers = false;
 var \$thisScriptUrl;
 function \$static_init(){};
 
 function \$initGlobals(context) {
   context.isolateStatics = new ${namer.ISOLATE}();
 }
 function \$setGlobals(context) {
   $currentIsolate = context.isolateStatics;
   \$globalThis = $currentIsolate;
 }
@@ skipping 117 matching lines @@
 const String HOOKS_API_USAGE = """
 // Generated by dart2js, the Dart to JavaScript compiler.
 // The code supports the following hooks:
 // dartPrint(message)   - if this function is defined it is called
 //                        instead of the Dart [print] method.
 // dartMainRunner(main) - if this function is defined, the Dart [main]
 //                        method will not be invoked directly.
 //                        Instead, a closure that will invoke [main] is
 //                        passed to [dartMainRunner].
 """;