Don't serialize Label in NamedExpression as a label declaration.

pkg/analyzer/lib/src/summary/summarize_ast.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 serialization.summarize_ast;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/src/generated/utilities_dart.dart';
@@ skipping 1121 matching lines @@
     b.isDeferred = node.deferredKeyword != null;
     b.uri = node.uri.stringValue;
     b.uriOffset = node.uri.offset;
     b.uriEnd = node.uri.end;
     unlinkedImports.add(b);
   }
 
   @override
   void visitLabel(Label node) {
     AstNode parent = node.parent;
-    labels.add(new UnlinkedLabelBuilder(
-        name: node.label.name,
-        nameOffset: node.offset,
-        isOnSwitchMember: parent is SwitchMember,
-        isOnSwitchStatement:
-            parent is LabeledStatement && parent.statement is SwitchStatement));
+    if (parent is! NamedExpression) {
+      labels.add(new UnlinkedLabelBuilder(
+          name: node.label.name,
+          nameOffset: node.offset,
+          isOnSwitchMember: parent is SwitchMember,
+          isOnSwitchStatement: parent is LabeledStatement &&
+              parent.statement is SwitchStatement));
+    }
   }
 
   @override
   void visitLibraryDirective(LibraryDirective node) {
     libraryName =
         node.name.components.map((SimpleIdentifier id) => id.name).join('.');
     libraryNameOffset = node.name.offset;
     libraryNameLength = node.name.length;
     isCoreLibrary = libraryName == 'dart.core';
     libraryDocumentationComment =
@@ skipping 77 matching lines @@
 /**
  * A [_TypeParameterScope] is a [_Scope] which defines [_ScopedTypeParameter]s.
  */
 class _TypeParameterScope extends _Scope {
   /**
    * Get the number of [_ScopedTypeParameter]s defined in this
    * [_TypeParameterScope].
    */
   int get length => _definedNames.length;
 }