Setup correct scope for initializers and complain about fields initialized more than once.

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 174 matching lines @@
       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 addFields(List<MetadataBuilder> metadata, int modifiers, T type,
-      List<Object> namesOffsetsAndInitializers) {
-    for (int i = 0; i < namesOffsetsAndInitializers.length; i += 4) {
-      String name = namesOffsetsAndInitializers[i];
-      int charOffset = namesOffsetsAndInitializers[i + 1];
-      Token initializer;
-      if (type == null) {
-        initializer = namesOffsetsAndInitializers[i + 2];
-        Token afterInitializer = namesOffsetsAndInitializers[i + 3];
-        // TODO(paulberry): figure out a way to do this without using
-        // Token.previous.
-        afterInitializer?.previous
-            ?.setNext(new Token.eof(afterInitializer.offset));
+      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) {
+        Token beforeLast = fieldsInfo[i + 3];
+        beforeLast.setNext(new Token.eof(beforeLast.next.offset));
       }
       addField(metadata, modifiers, type, name, charOffset, initializer);
     }
   }
 
   void addProcedure(
       List<MetadataBuilder> metadata,
       int modifiers,
       T returnType,
       String name,
@@ skipping 361 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);
   }
 }