Add UnlinkedConst.isValid and set it during summarizing.

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

Index: pkg/analyzer/lib/src/summary/summarize_elements.dart
diff --git a/pkg/analyzer/lib/src/summary/summarize_elements.dart b/pkg/analyzer/lib/src/summary/summarize_elements.dart
index 92ea9a44179c19662ff5e392f8bd7ce2fe52b55f..3bda047e7f070999ef5ea64a1089aedde64bcb24 100644
--- a/pkg/analyzer/lib/src/summary/summarize_elements.dart
+++ b/pkg/analyzer/lib/src/summary/summarize_elements.dart
@@ -438,8 +438,7 @@ class _CompilationUnitSerializer {
    */
   UnlinkedConstBuilder serializeConstExpr(Expression expression) {
     _ConstExprSerializer serializer = new _ConstExprSerializer(this);
-    serializer.serialize(expression);
-    return serializer.toBuilder();
+    return serializer.serialize(expression);
   }
 
   /**
@@ -945,7 +944,9 @@ class _ConstExprSerializer extends AbstractConstExprSerializer {
     if (identifier is PrefixedIdentifier &&
         element is PropertyAccessorElement &&
         !element.isStatic) {
-      assert(element.name == 'length');
+      if (element.name != 'length') {
+        throw new StateError('Only "length" property is allowed in constants.');
+      }
       Element prefixElement = identifier.prefix.staticElement;
       int prefixRef = serializer._getElementReferenceId(prefixElement);
       int lengthRef = serializer._getLengthPropertyReference(prefixRef);