Create an option to disable type inference and type promotion.

pkg/front_end/lib/src/fasta/kernel/kernel_field_builder.dart

 // Copyright (c) 2016, 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 fasta.kernel_field_builder;
 
 import 'package:front_end/src/fasta/kernel/body_builder.dart' show BodyBuilder;
 
 import 'package:front_end/src/fasta/kernel/kernel_shadow_ast.dart'
     show KernelField;
@@ skipping 54 matching lines @@
     if (type != null) {
       field.type = type.build(library);
     }
     bool isInstanceMember = !isStatic && !isTopLevel;
     field
       ..isFinal = isFinal
       ..isConst = isConst
       ..hasImplicitGetter = isInstanceMember
       ..hasImplicitSetter = isInstanceMember && !isConst && !isFinal
       ..isStatic = !isInstanceMember;
-    if (isEligibleForInference) {
+    if (!library.disableTypeInference && isEligibleForInference) {
       library.loader.typeInferenceEngine.recordMember(field);
     }
     return field;
   }
 
   Field get target => field;
 
   @override
   void prepareInitializerInference(
       SourceLibraryBuilder library, ClassBuilder currentClass) {
-    if (isEligibleForInference) {
+    if (!library.disableTypeInference && isEligibleForInference) {
       var memberScope =
           currentClass == null ? library.scope : currentClass.scope;
       var typeInferenceEngine = library.loader.typeInferenceEngine;
       var listener = new TypeInferenceListener();
       var typeInferrer = typeInferenceEngine.createTopLevelTypeInferrer(
           listener, field.enclosingClass?.thisType, field);
       if (hasInitializer) {
         var bodyBuilder = new BodyBuilder(
             library,
             this,
@@ skipping 13 matching lines @@
       }
     }
   }
 
   @override
   DartType get builtType => field.type;
 
   @override
   bool get hasImplicitType => type == null;
 }