Add type annotations to remove warnings and fix a bug.

lib/compiler/implementation/dart_backend/backend.dart

Index: lib/compiler/implementation/dart_backend/backend.dart
diff --git a/lib/compiler/implementation/dart_backend/backend.dart b/lib/compiler/implementation/dart_backend/backend.dart
index 45eef3fd8c951a3f3a7e5e659df9814f174f7570..3cd8a32dcd0c2a1500038b3d0fd4626f2932cdf7 100644
--- a/lib/compiler/implementation/dart_backend/backend.dart
+++ b/lib/compiler/implementation/dart_backend/backend.dart
@@ -326,7 +326,7 @@ class DartBackend extends Backend {
         processElement(element, elementAst);
       } else {
         if (!element.isTopLevel()) {
-          compiler.cancel(reason: 'Cannot process $element', element: element);
+          compiler.cancel('Cannot process $element', element: element);
         }
         addTopLevel(element, elementAst);
       }