Minor fixes to for-loop type inference

pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
index 1293511b19a8f154d576f5671d16478e508c4ee5..8957ac05ead5ba8d16e2cea1c354f43854235e55 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
@@ -759,8 +759,10 @@ class KernelForStatement extends ForStatement implements KernelStatement {
   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);
     }