Fix dart2js warnings and add test to ensure it stays clean.

pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart

 // Copyright (c) 2014, 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.js_emitter.program_builder;
 
 import '../../closure.dart' show ClosureFieldElement;
 import '../../common.dart';
 import '../../common/names.dart' show Names, Selectors;
 import '../../compiler.dart' show Compiler;
@@ skipping 379 matching lines @@
             if (member.isFunction) {
               FunctionElement fn = member;
               functionType = fn.type;
             } else if (member.isGetter) {
               if (_compiler.options.trustTypeAnnotations) {
                 GetterElement getter = member;
                 ResolutionDartType returnType = getter.type.returnType;
                 if (returnType.isFunctionType) {
                   functionType = returnType;
                 } else if (returnType.treatAsDynamic ||
-                    _compiler.types.isSubtype(returnType,
+                    _compiler.types.isSubtype(
+                        returnType,
+                        // ignore: UNNECESSARY_CAST
                         backend.commonElements.functionType as DartType)) {
                   if (returnType.isTypedef) {
                     ResolutionTypedefType typedef = returnType;
                     // TODO(jacobr): can we just use typdef.unaliased instead?
                     functionType = typedef.element.functionSignature.type;
                   } else {
                     // Other misc function type such as commonElements.Function.
                     // Allow any number of arguments.
                     isFunctionLike = true;
                   }
@@ skipping 559 matching lines @@
       Constant constant = new Constant(name, holder, constantValue);
       _constants[constantValue] = constant;
     }
   }
 
   Holder _registerStaticStateHolder() {
     return _registry.registerHolder(namer.staticStateHolder,
         isStaticStateHolder: true);
   }
 }