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

pkg/compiler/lib/src/js_emitter/program_builder/program_builder.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.program_builder;
 
 import '../../closure.dart' show ClosureFieldElement;
 import '../../common.dart';
 import '../../common/names.dart' show Names, Selectors;
 import '../../compiler.dart' show Compiler;
@@ skipping 12 matching lines @@
         FunctionElement,
         FunctionSignature,
         GetterElement,
         LibraryElement,
         MemberElement,
         MethodElement,
         ParameterElement,
         TypedefElement,
         VariableElement;
 import '../../elements/entities.dart';
-import '../../elements/types.dart' show DartType;
 import '../../js/js.dart' as js;
 import '../../js_backend/backend_helpers.dart' show BackendHelpers;
 import '../../js_backend/js_backend.dart'
     show Namer, JavaScriptBackend, JavaScriptConstantCompiler, StringBackedName;
 import '../../universe/selector.dart' show Selector;
 import '../../universe/world_builder.dart'
     show CodegenWorldBuilder, SelectorConstraints;
 import '../../world.dart' show ClosedWorld;
 import '../js_emitter.dart'
     show
@@ skipping 346 matching lines @@
             if (member.isFunction) {
               FunctionElement fn = member;
               functionType = fn.type;
             } else if (member.isGetter) {
               if (_compiler.options.trustTypeAnnotations) {
                 GetterElement getter = member;
                 ResolutionDartType returnType = getter.type.returnType;
                 if (returnType.isFunctionType) {
                   functionType = returnType;
                 } else if (returnType.treatAsDynamic ||
-                    _compiler.types.isSubtype(returnType,
-                        backend.commonElements.functionType as DartType)) {
+                    _compiler.types.isSubtype(
+                        returnType,
+                        // ignore: ARGUMENT_TYPE_NOT_ASSIGNABLE
+                        backend.commonElements.functionType)) {

[Johnni Winther, 2017/04/03 12:07:33]

Reinsert the cast and ignore the hint.

[ahe, 2017/04/03 12:25:07]

Done.

                   if (returnType.isTypedef) {
                     ResolutionTypedefType typedef = returnType;
                     // TODO(jacobr): can we just use typdef.unaliased instead?
                     functionType = typedef.element.functionSignature.type;
                   } else {
                     // Other misc function type such as commonElements.Function.
                     // Allow any number of arguments.
                     isFunctionLike = true;
                   }
                 }
@@ skipping 558 matching lines @@
       Constant constant = new Constant(name, holder, constantValue);
       _constants[constantValue] = constant;
     }
   }
 
   Holder _registerStaticStateHolder() {
     return _registry.registerHolder(namer.staticStateHolder,
         isStaticStateHolder: true);
   }
 }