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

pkg/compiler/lib/src/ssa/nodes.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 '../closure.dart';
 import '../common.dart';
 import '../common/backend_api.dart' show BackendClasses;
 import '../compiler.dart' show Compiler;
 import '../constants/constant_system.dart';
 import '../constants/values.dart';
@@ skipping 3373 matching lines @@
 
   int typeCode() => HInstruction.TYPE_INFO_EXPRESSION_TYPECODE;
   bool typeEquals(HInstruction other) => other is HTypeInfoExpression;
 
   bool dataEquals(HTypeInfoExpression other) {
     return kind == other.kind && dartType == other.dartType;
   }
 
   String toString() => 'HTypeInfoExpression($kindAsString, $dartType)';
 
+  // ignore: MISSING_RETURN
   String get kindAsString {
     switch (kind) {
       case TypeInfoExpressionKind.COMPLETE:
         return 'COMPLETE';
       case TypeInfoExpressionKind.INSTANCE:
         return 'INSTANCE';
     }
   }
 }