Minor fixes to for-loop type inference

pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.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.
 
 /// This file declares a "shadow hierarchy" of concrete classes which extend
 /// the kernel class hierarchy, adding methods and fields needed by the
 /// BodyBuilder.
 ///
 /// Instances of these classes may be created using the factory methods in
 /// `ast_factory.dart`.
@@ skipping 741 matching lines @@
 /// Concrete shadow object representing a classic for loop in kernel form.
 class KernelForStatement extends ForStatement implements KernelStatement {
   KernelForStatement(List<VariableDeclaration> variables, Expression condition,
       List<Expression> updates, Statement body)
       : super(variables, condition, updates, body);
 
   @override
   void _inferStatement(KernelTypeInferrer inferrer) {
     inferrer.listener.forStatementEnter(this);
     variables.forEach(inferrer.inferStatement);
-    inferrer.inferExpression(
-        condition, inferrer.coreTypes.boolClass.rawType, false);
+    if (condition != null) {
+      inferrer.inferExpression(
+          condition, inferrer.coreTypes.boolClass.rawType, false);
+    }
     for (var update in updates) {
       inferrer.inferExpression(update, null, false);
     }
     inferrer.inferStatement(body);
     inferrer.listener.forStatementExit(this);
   }
 }
 
 /// Concrete shadow object representing a local function declaration in kernel
 /// form.
@@ skipping 1643 matching lines @@
   }
 
   transformChildren(v) {
     return internalError("Internal error: Unsupported operation.");
   }
 
   visitChildren(v) {
     return internalError("Internal error: Unsupported operation.");
   }
 }