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

pkg/front_end/lib/src/fasta/source/source_library_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.source_library_builder;
 
 import 'package:front_end/src/scanner/token.dart' show Token;
 
 import 'package:kernel/ast.dart' show ProcedureKind;
 
@@ skipping 170 matching lines @@
 
   void addNamedMixinApplication(
       List<MetadataBuilder> metadata,
       String name,
       List<TypeVariableBuilder> typeVariables,
       int modifiers,
       T mixinApplication,
       List<T> interfaces,
       int charOffset);
 
-  void addField(List<MetadataBuilder> metadata, int modifiers, T type,
-      String name, int charOffset, Token initializer);
+  void addField(
+      List<MetadataBuilder> metadata,
+      int modifiers,
+      T type,
+      String name,
+      int charOffset,
+      Token initializerTokenForInference,
+      bool hasInitializer);
 
   void addFields(List<MetadataBuilder> metadata, int modifiers, T type,
       List<Object> fieldsInfo) {
     for (int i = 0; i < fieldsInfo.length; i += 4) {
       String name = fieldsInfo[i];
       int charOffset = fieldsInfo[i + 1];
-      Token initializer = fieldsInfo[i + 2];
-      if (initializer != null) {
+      bool hasInitializer = fieldsInfo[i + 2] != null;
+      Token initializerTokenForInference =
+          type == null ? fieldsInfo[i + 2] : null;
+      if (initializerTokenForInference != null) {
         Token beforeLast = fieldsInfo[i + 3];
         beforeLast.setNext(new Token.eof(beforeLast.next.offset));
       }
-      addField(metadata, modifiers, type, name, charOffset, initializer);
+      addField(metadata, modifiers, type, name, charOffset,
+          initializerTokenForInference, hasInitializer);
     }
   }
 
   void addProcedure(
       List<MetadataBuilder> metadata,
       int modifiers,
       T returnType,
       String name,
       List<TypeVariableBuilder> typeVariables,
       List<FormalParameterBuilder> formals,
@@ skipping 375 matching lines @@
   /// synthesize type variables on the factory matching the class'.
   void addFactoryDeclaration(
       ProcedureBuilder procedure, DeclarationBuilder<T> factoryDeclaration) {
     factoryDeclarations[procedure] = factoryDeclaration;
   }
 
   Scope toScope(Scope parent) {
     return new Scope(members, setters, parent, isModifiable: false);
   }
 }