fix #25220, infer generic type from constructor arguments

pkg/analyzer/lib/src/generated/resolver.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.
 
 library analyzer.src.generated.resolver;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 6705 matching lines @@
   }
 
   @override
   Object visitInstanceCreationExpression(InstanceCreationExpression node) {
     TypeName classTypeName = node.constructorName.type;
     if (classTypeName.typeArguments == null) {
       DartType contextType = InferenceContext.getType(node);
       if (contextType is InterfaceType &&
           contextType.typeArguments != null &&
           contextType.typeArguments.length > 0) {
+        // TODO(jmesserly): for generic methods we use the
+        // StrongTypeSystemImpl.inferGenericFunctionCall, which appears to
+        // be a tad more powerful than matchTypes.
+        //
+        // For example it can infer this case:
+        //
+        //     class E<S, T> extends A<C<S>, T> { ... }
+        //     A<C<int>, String> a0 = /*infer<int, String>*/new E("hello");
+        //
+        // See _inferArgumentTypesFromContext in this file for use of it.
         List<DartType> targs =
             inferenceContext.matchTypes(classTypeName.type, contextType);
         if (targs != null && targs.any((t) => !t.isDynamic)) {
           ClassElement classElement = classTypeName.type.element;
           InterfaceType rawType = classElement.type;
           InterfaceType fullType =
               rawType.substitute2(targs, rawType.typeArguments);
           // The element resolver uses the type on the constructor name, so
           // infer it first
           typeAnalyzer.inferConstructorName(node.constructorName, fullType);
@@ skipping 4304 matching lines @@
       return null;
     }
     if (identical(node.staticElement, variable)) {
       if (node.inSetterContext()) {
         result = true;
       }
     }
     return null;
   }
 }