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

pkg/compiler/lib/src/kernel/kernel_visitor.dart

 // Copyright (c) 2016, 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.md file.
 
 import 'package:kernel/ast.dart' as ir;
 import 'package:kernel/frontend/accessors.dart'
     show
         Accessor,
         IndexAccessor,
         NullAwarePropertyAccessor,
@@ skipping 335 matching lines @@
         receiver, nameToIrName(selector.memberName), arguments);
   }
 
   ir.MethodInvocation buildCall(
       ir.Expression receiver, CallStructure callStructure, NodeList arguments) {
     return buildInvokeSelector(
         receiver, callStructure.callSelector, buildArguments(arguments));
   }
 
   @override
+  // ignore: INVALID_METHOD_OVERRIDE_RETURN_TYPE
   ir.Expression visitIdentifier(Identifier node) {
     // TODO(ahe): Shouldn't have to override this method, but
     // [SemanticSendResolvedMixin.visitIdentifier] may return `null` on errors.
     if (node.isThis()) {
       return sendVisitor.visitThisGet(node, null);
     } else {
       return new ir.InvalidExpression();
     }
   }
 
@@ skipping 1433 matching lines @@
 
   ir.VariableGet buildLocalGet(LocalElement local) {
     return new ir.VariableGet(getLocal(local));
   }
 
   @override
   ir.VariableGet handleLocalGet(Send node, LocalElement element, _) {
     return buildLocalGet(element);
   }
 
+  // ignore: MISSING_RETURN
   ir.Expression buildCompound(
       Accessor accessor, CompoundRhs rhs, SendSet node) {
     ir.Name name = kernel.irName(rhs.operator.selectorName, currentElement);
     switch (rhs.kind) {
       case CompoundKind.POSTFIX:
         return buildPostfixIncrement(node, accessor, name);
 
       case CompoundKind.PREFIX:
         return buildPrefixIncrement(node, accessor, name);
 
@@ skipping 1049 matching lines @@
   }
 
   @override
   ir.Expression visitSuperIndexSetIfNull(SendSet node, MethodElement getter,
       MethodElement setter, Node index, Node rhs, _) {
     return _finishSetIfNull(
         node, buildSuperIndexAccessor(index, getter, setter), rhs);
   }
 
   @override
+  // ignore: INVALID_METHOD_OVERRIDE_RETURN_TYPE
   ir.Node visitVariableDefinitions(VariableDefinitions definitions) {
     // TODO(ahe): This method is copied from [SemanticDeclarationResolvedMixin]
     // and modified. Perhaps we can find a way to avoid code duplication.
     List<ir.VariableDeclaration> variables = <ir.VariableDeclaration>[];
     computeVariableStructures(definitions,
         (Node node, VariableStructure structure) {
       if (structure == null) {
         return internalError(node, 'No structure for $node');
       } else {
         ir.VariableDeclaration variable =
@@ skipping 75 matching lines @@
       : this(null, true, node, initializers);
 
   accept(ir.Visitor v) => throw "unsupported";
 
   visitChildren(ir.Visitor v) => throw "unsupported";
 
   String toString() {
     return "IrFunction($kind, $isConstructor, $node, $initializers)";
   }
 }