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

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

Index: pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart
diff --git a/pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart b/pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart
index 63448b83172b3def29be45248238e0facd05aa85..652281af0f2183fc6210a9f9d0d2454ba61581de 100644
--- a/pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart
+++ b/pkg/compiler/lib/src/js_emitter/program_builder/program_builder.dart
@@ -30,7 +30,6 @@ import '../../elements/elements.dart'
         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'
@@ -397,8 +396,10 @@ class ProgramBuilder {
                 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?