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

pkg/compiler/lib/src/ssa/builder.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.
 
 import 'dart:collection';
 
 import 'package:js_runtime/shared/embedded_names.dart';
 
 import '../closure.dart';
 import '../common.dart';
@@ skipping 620 matching lines @@
     ConstantValue constantValue =
         backend.constants.getConstantValue(parameter.constant);
     assert(invariant(parameter, constantValue != null,
         message: 'No constant computed for $parameter'));
     return graph.addConstant(constantValue, closedWorld);
   }
 
   ClassElement get currentNonClosureClass {
     ClassElement cls = sourceElement.enclosingClass;
     if (cls != null && cls.isClosure) {
-      var closureClass = cls;
+      dynamic closureClass = cls;
+      // ignore: UNDEFINED_GETTER
       return closureClass.methodElement.enclosingClass;
     } else {
       return cls;
     }
   }
 
   /// A stack of [ResolutionDartType]s that have been seen during inlining of
   /// factory constructors.  These types are preserved in [HInvokeStatic]s and
   /// [HCreate]s inside the inline code and registered during code generation
   /// for these nodes.
@@ skipping 6107 matching lines @@
       this.oldReturnLocal,
       this.oldReturnType,
       this.oldResolvedAst,
       this.oldStack,
       this.oldLocalsHandler,
       this.inTryStatement,
       this.allFunctionsCalledOnce,
       this.oldElementInferenceResults)
       : super(function);
 }