Fix some warnings.

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 534 matching lines @@
 
   String generateCheckedSetter(Element member, String fieldName) {
     DartType type = member.computeType(compiler);
     if (type.element.isTypeVariable()
         || type.element == compiler.dynamicClass
         || type.element == compiler.objectClass) {
       // TODO(ngeoffray): Support type checks on type parameters.
       return null;
     } else {
       SourceString helper = compiler.backend.getCheckedModeHelper(type);
-      Element helperElement = compiler.findHelper(helper);
+      FunctionElement helperElement = compiler.findHelper(helper);
       String helperName = namer.isolateAccess(helperElement);
       String additionalArgument = '';
       if (helperElement.computeSignature(compiler).parameterCount != 1) {
         additionalArgument = ", '${namer.operatorIs(type.element)}'";
       }
       return " set\$$fieldName: function(v) { "
           "this.$fieldName = $helperName(v$additionalArgument); }";
     }
   }
 
@@ skipping 97 matching lines @@
     }
 
     classElement.implementation.forEachMember(includeBackendMembers: true,
         f: (ClassElement enclosing, Element member) {
       assert(invariant(classElement, member.isDeclaration));
       if (member.isInstanceMember()) {
         addInstanceMember(member, defineInstanceMember);
       }
     });
 
-    generateIsTestsOn(classElement, (Element other) {
+    generateIsTestsOn(classElement, (ClassElement other) {
       String code;
       if (other.isObject(compiler)) return;
       if (nativeEmitter.requiresNativeIsCheck(other)) {
         code = 'function() { return true; }';
       } else {
         code = 'true';
       }
       CodeBuffer typeTestBuffer = new CodeBuffer();
       typeTestBuffer.add(code);
       defineInstanceMember(namer.operatorIs(other), typeTestBuffer);
@@ skipping 751 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].
 """;