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

pkg/analyzer/lib/src/summary/summarize_ast.dart

Index: pkg/analyzer/lib/src/summary/summarize_ast.dart
diff --git a/pkg/analyzer/lib/src/summary/summarize_ast.dart b/pkg/analyzer/lib/src/summary/summarize_ast.dart
index b268f3d1dc9262fcf18018a1b8fb64e30c65c462..715267bde9d6f8e81af3d2ab7f2b08175efb27c4 100644
--- a/pkg/analyzer/lib/src/summary/summarize_ast.dart
+++ b/pkg/analyzer/lib/src/summary/summarize_ast.dart
@@ -1139,12 +1139,14 @@ class _SummarizeAstVisitor extends RecursiveAstVisitor {
   @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