Add type inference logic for "for-in" loops.

pkg/analyzer/test/src/task/strong/front_end_inference_test.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
@@ skipping 386 matching lines @@
       }
     }
 
     if (element is LocalVariableElement) {
       recordPromotions(element.type);
     } else if (element is ParameterElement) {
       recordPromotions(element.type);
     }
   }
 
+  @override
+  visitDeclaredIdentifier(DeclaredIdentifier node) {
+    super.visitDeclaredIdentifier(node);
+    if (node.type == null) {
+      _recordType(node.identifier.offset, node.element.type);
+    }
+  }
+
   visitVariableDeclarationList(VariableDeclarationList node) {
     super.visitVariableDeclarationList(node);
     if (node.type == null) {
       for (VariableDeclaration variable in node.variables) {
         VariableElement element = variable.element;
         if (element is LocalVariableElement) {
           _recordType(variable.name.offset, element.type);
         } else {
           _recordTopType(variable.name.offset, element.type);
         }
@@ skipping 38 matching lines @@
 
   /// Based on DDC code generator's `_recoverTypeArguments`
   Iterable<DartType> _recoverTypeArguments(FunctionType g, FunctionType f) {
     assert(identical(g.element, f.element));
     assert(g.typeFormals.isNotEmpty && f.typeFormals.isEmpty);
     assert(g.typeFormals.length + g.typeArguments.length ==
         f.typeArguments.length);
     return f.typeArguments.skip(g.typeArguments.length);
   }
 }