Only attempt to do type inference on fields that lack a declared type.

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 16 matching lines @@
 
 import '../errors.dart' show internalError;
 
 import 'kernel_builder.dart'
     show Builder, FieldBuilder, KernelTypeBuilder, MetadataBuilder;
 
 class KernelFieldBuilder extends FieldBuilder<Expression> {
   final Field field;
   final List<MetadataBuilder> metadata;
   final KernelTypeBuilder type;
-  final Token initializerToken;
+  final Token initializerTokenForInference;
+  final bool hasInitializer;
 
-  KernelFieldBuilder(this.metadata, this.type, String name, int modifiers,
-      Builder compilationUnit, int charOffset, this.initializerToken)
+  KernelFieldBuilder(
+      this.metadata,
+      this.type,
+      String name,
+      int modifiers,
+      Builder compilationUnit,
+      int charOffset,
+      this.initializerTokenForInference,
+      this.hasInitializer)
       : field = new KernelField(null, fileUri: compilationUnit?.relativeFileUri)
           ..fileOffset = charOffset,
         super(name, modifiers, compilationUnit, charOffset);
 
-  bool get hasInitializer => initializerToken != null;
-
   void set initializer(Expression value) {
     if (!hasInitializer && value is! NullLiteral && !isConst && !isFinal) {
       internalError("Attempt to set initializer on field without initializer.");
     }
     field.initializer = value..parent = field;
   }
 
   Field build(SourceLibraryBuilder library) {
     field.name ??= new Name(name, library.target);
     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 (initializerToken != null && !initializerToken.isEof) {
+    if (initializerTokenForInference != null &&
+        !initializerTokenForInference.isEof) {
+      assert(type == null);
       library.loader.typeInferenceEngine.recordField(field);
     }
     return field;
   }
 
   Field get target => field;
 
   @override
   void prepareInitializerInference(
       SourceLibraryBuilder library, ClassBuilder currentClass) {
-    if (initializerToken != null && !initializerToken.isEof) {
+    if (initializerTokenForInference != null &&
+        !initializerTokenForInference.isEof) {
       var memberScope =
           currentClass == null ? library.scope : currentClass.scope;
       // TODO(paulberry): Is it correct to pass library.uri into BodyBuilder, or
       // should it be the part URI?
       var typeInferenceEngine = library.loader.typeInferenceEngine;
       var listener = new TypeInferenceListener();
       var typeInferrer = typeInferenceEngine.createTopLevelTypeInferrer(
           listener, field.enclosingClass?.thisType, field);
       var bodyBuilder = new BodyBuilder(
           library,
           this,
           memberScope,
           null,
           typeInferenceEngine.classHierarchy,
           typeInferenceEngine.coreTypes,
           currentClass,
           isInstanceMember,
           library.uri,
           typeInferrer);
       Parser parser = new Parser(bodyBuilder);
-      Token token = parser.parseExpression(initializerToken);
+      Token token = parser.parseExpression(initializerTokenForInference);
       Expression expression = bodyBuilder.popForValue();
       bodyBuilder.checkEmpty(token.charOffset);
       initializer = expression;
     }
   }
 
   @override
   DartType get builtType => field.type;
 }