fix #25794, infer parameter type from default value

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

 // Copyright (c) 2015, 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.elements;
 
 import 'dart:convert';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
@@ skipping 884 matching lines @@
       case ParameterKind.NAMED:
         b.kind = UnlinkedParamKind.named;
         break;
     }
     b.annotations = serializeAnnotations(parameter);
     b.codeRange = serializeCodeRange(parameter);
     b.isInitializingFormal = parameter.isInitializingFormal;
     DartType type = parameter.type;
     if (parameter.hasImplicitType) {
       Element contextParent = context.enclosingElement;
+      // Strong mode infers parameters in two cases:
+      // - instance members (i.e. not constructors or static members),
+      // - parameters with default values, except initializing formals
+      //   (the type comes from the field).
       if (!parameter.isInitializingFormal &&
           contextParent is ExecutableElement &&
-          !contextParent.isStatic &&
-          contextParent is! ConstructorElement) {
+          (!contextParent.isStatic && contextParent is! ConstructorElement ||
+              parameter.parameterKind != ParameterKind.REQUIRED)) {
         b.inferredTypeSlot = storeInferredType(type, context);
       }
     } else {
       if (type is FunctionType && type.element.isSynthetic) {
         b.isFunctionTyped = true;
         b.type = serializeTypeRef(type.returnType, parameter);
         b.parameters = type.parameters
             .map((parameter) => serializeParam(parameter, context))
             .toList();
       } else {
@@ skipping 756 matching lines @@
       exportNames.add(new LinkedExportNameBuilder(
           name: name,
           dependency: serializeDependency(dependentLibrary),
           unit: unit,
           kind: kind));
     }
     pb.exportNames = exportNames;
     return pb;
   }
 }