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

pkg/compiler/lib/src/js_emitter/full_emitter/setup_program_builder.dart

 // Copyright (c) 2013, 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.
 
 part of dart2js.js_emitter.full_emitter;
 
 // TODO(ahe): Share these with js_helper.dart.
 const FUNCTION_INDEX = 0;
 const NAME_INDEX = 1;
 const CALL_NAME_INDEX = 2;
@@ skipping 114 matching lines @@
     'needsMixinSupport': emitter.needsMixinSupport,
     'needsNativeSupport': program.needsNativeSupport,
     'enabledJsInterop': backend.jsInteropAnalysis.enabledJsInterop,
     'jsInteropBoostrap': backend.jsInteropAnalysis.buildJsInteropBootstrap(),
     'isInterceptorClass': namer.operatorIs(backend.helpers.jsInterceptorClass),
     'isObject': namer.operatorIs(compiler.commonElements.objectClass),
     'specProperty': js.string(namer.nativeSpecProperty),
     'trivialNsmHandlers': emitter.buildTrivialNsmHandlers(),
     'hasRetainedMetadata': backend.mirrorsData.hasRetainedMetadata,
     'types': typesAccess,
-    'objectClassName': js.quoteName(
-        namer.runtimeTypeName(compiler.commonElements.objectClass as Entity)),
+    'objectClassName': js.quoteName(namer.runtimeTypeName(
+        // ignore: UNNECESSARY_CAST
+        compiler.commonElements.objectClass as Entity)),
     'needsStructuredMemberInfo': emitter.needsStructuredMemberInfo,
     'usesMangledNames': compiler.commonElements.mirrorsLibrary != null ||
         backend.backendUsage.isFunctionApplyUsed,
     'tearOffCode': buildTearOffCode(backend),
     'nativeInfoHandler': nativeInfoHandler,
     'operatorIsPrefix': js.string(namer.operatorIsPrefix),
     'deferredActionString': js.string(namer.deferredAction)
   };
   String skeleton = '''
 function $setupProgramName(programData, typesOffset) {
@@ skipping 694 matching lines @@
 (function() {
   var result = $array[$index];
   if ($check) {
     throw new Error(
         name + ": expected value of type \'$type\' at index " + ($index) +
         " but got " + (typeof result));
   }
   return result;
 })()''';
 }