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

pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart

 // Copyright (c) 2014, 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.
 
 library dart2js.js_emitter.runtime_type_generator;
 
 import '../closure.dart' show ClosureClassMap, ClosureFieldElement;
 import '../common.dart';
 import '../common/names.dart' show Identifiers;
 import '../compiler.dart' show Compiler;
 import '../common_elements.dart' show CommonElements;
 import '../elements/resolution_types.dart'
-    show ResolutionDartType, ResolutionFunctionType, ResolutionTypeVariableType;
+    show ResolutionDartType, ResolutionFunctionType;
 import '../elements/elements.dart'
-    show
-        ClassElement,
-        Element,
-        FunctionElement,
-        MixinApplicationElement,
-        TypeVariableElement;
+    show ClassElement, Element, FunctionElement, MixinApplicationElement;
 import '../js/js.dart' as jsAst;
 import '../js/js.dart' show js;
 import '../js_backend/js_backend.dart'
     show
         JavaScriptBackend,
         Namer,
         RuntimeTypesEncoder,
         RuntimeTypesSubstitutions,
         Substitution,
         TypeCheck,
         TypeChecks;
 import '../util/util.dart' show Setlet;
 
 import 'code_emitter_task.dart' show CodeEmitterTask;
-import 'model.dart';
 import 'type_test_registry.dart' show TypeTestRegistry;
 
 // Function signatures used in the generation of runtime type information.
 typedef void FunctionTypeSignatureEmitter(
     Element method, ResolutionFunctionType methodType);
 
 typedef void SubstitutionEmitter(Element element, {bool emitNull});
 
 class TypeTestProperties {
   /// The index of the function type into the metadata.
@@ skipping 277 matching lines @@
 
     // We need to also emit "is checks" for the superclass and its supertypes.
     ClassElement superclass = cls.superclass;
     if (superclass != null) {
       tryEmitTest(superclass);
       _generateInterfacesIsTests(
           superclass, generateIsTest, generateSubstitution, alreadyGenerated);
     }
   }
 }