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

pkg/compiler/lib/src/inferrer/inferrer_engine.dart

 // Copyright (c) 2017, 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 'package:kernel/ast.dart' as ir;
 
 import '../common.dart';
 import '../common/names.dart';
 import '../compiler.dart';
 import '../constants/expressions.dart';
@@ skipping 456 matching lines @@
     reporter.log('Inferred $overallRefineCount types.');
 
     processLoopInformation();
   }
 
   void analyze(ResolvedAst resolvedAst, ArgumentsTypes arguments) {
     AstElement element = resolvedAst.element.implementation;
     if (analyzedElements.contains(element)) return;
     analyzedElements.add(element);
 
-    var visitor = compiler.options.kernelGlobalInference
+    dynamic visitor = compiler.options.kernelGlobalInference
         ? new KernelTypeGraphBuilder(element, resolvedAst, compiler, this)
         : new ElementGraphBuilder(element, resolvedAst, compiler, this);
     TypeInformation type;
     reporter.withCurrentElement(element, () {
+      // ignore: UNDEFINED_METHOD
       type = visitor.run();
     });
     addedInGraph++;
 
     if (element.isField) {
       VariableElement fieldElement = element;
       ast.Node node = resolvedAst.node;
       ast.Node initializer = resolvedAst.body;
       if (element.isFinal || element.isConst) {
         // If [element] is final and has an initializer, we record
@@ skipping 564 matching lines @@
   /**
    * Records that the captured variable [local] is read.
    */
   void recordCapturedLocalRead(Local local) {}
 
   /**
    * Records that the variable [local] is being updated.
    */
   void recordLocalUpdate(Local local, TypeInformation type) {}
 }