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

pkg/compiler/lib/src/kernel/kernel.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 'dart:collection' show Queue;
 
 import 'package:kernel/ast.dart' as ir;
 import 'package:kernel/verifier.dart' show CheckParentPointers;
 
 import '../common.dart';
@@ skipping 333 matching lines @@
   }
 
   List<ir.Supertype> supertypesToIr(List<ResolutionDartType> types) {
     List<ir.Supertype> result = new List<ir.Supertype>(types.length);
     for (int i = 0; i < types.length; i++) {
       result[i] = supertypeToIr(types[i]);
     }
     return result;
   }
 
+  // ignore: MISSING_RETURN
   ir.DartType typeToIr(ResolutionDartType type) {
     switch (type.kind) {
       case ResolutionTypeKind.FUNCTION:
         return functionTypeToIr(type);
 
       case ResolutionTypeKind.INTERFACE:
         return interfaceTypeToIr(type);
 
       case ResolutionTypeKind.TYPEDEF:
         type.computeUnaliased(compiler.resolution);
@@ skipping 415 matching lines @@
 }
 
 class ConstructorTarget {
   final ConstructorElement element;
   final ResolutionDartType type;
 
   ConstructorTarget(this.element, this.type);
 
   String toString() => "ConstructorTarget($element, $type)";
 }