Don't visit named argument labels when computing constant dependencies.

pkg/analyzer/lib/src/generated/constant.dart

 // Copyright (c) 2014, 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 code was auto-generated, is not intended to be edited, and is subject to
 // significant change. Please see the README file for more information.
 
 library engine.constant;
 
 import 'dart:collection';
@@ skipping 4988 matching lines @@
       ConstructorElement constructor =
           ConstantEvaluationEngine._getConstructorBase(node.staticElement);
       if (constructor != null) {
         _callback(constructor);
       }
     }
     return super.visitInstanceCreationExpression(node);
   }
 
   @override
+  Object visitLabel(Label node) {
+    // We are visiting the "label" part of a named expression in a function
+    // call (presumably a constructor call), e.g. "const C(label: ...)".  We
+    // don't want to visit the SimpleIdentifier for the label because that's a
+    // reference to a function parameter that needs to be filled in; it's not a
+    // constant whose value we depend on.
+    return null;
+  }
+
+  @override
   Object visitRedirectingConstructorInvocation(
       RedirectingConstructorInvocation node) {
     super.visitRedirectingConstructorInvocation(node);
     ConstructorElement target =
         ConstantEvaluationEngine._getConstructorBase(node.staticElement);
     if (target != null) {
       _callback(target);
     }
     return null;
   }
@@ skipping 231 matching lines @@
       return BoolState.from(_element == rightElement);
     } else if (rightOperand is DynamicState) {
       return BoolState.UNKNOWN_VALUE;
     }
     return BoolState.FALSE_STATE;
   }
 
   @override
   String toString() => _element == null ? "-unknown-" : _element.name;
 }