fixes #27586, prefer context type in generic inference

pkg/dev_compiler/lib/src/compiler/code_generator.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.
 
 import 'dart:collection' show HashMap, HashSet;
 import 'dart:math' show min, max;
 
 import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 5344 matching lines @@
     // TODO(jmesserly): verify this is unreachable.
     throw 'Unimplemented ${node.runtimeType}: $node';
   }
 
   _visit(AstNode node) {
     if (node == null) return null;
     var result = node.accept(this);
     return result is JS.Node ? annotate(result, node) : result;
   }
 
-  List/*<T>*/ _visitList/*<T extends AstNode>*/(Iterable/*<T>*/ nodes) {
+  // TODO(jmesserly): we should make sure this only returns JS AST nodes.
+  List/*<T>*/ _visitList/*<T>*/(Iterable nodes) {

[vsm, 2016/11/30 04:05:33]

Why was this change needed?

[Jennifer Messerly, 2016/11/30 04:35:04]

the type was completely wrong before. It's mixing up Dart AST nodes and JS AST
nodes. The return type definitely is not the same as the input type. Possibly we
could have two generic types T and R.

To be honest, DDC has some typing sloppiness in what its visit methods do, it
would be great to clean up at some point.

     if (nodes == null) return null;
     var result = /*<T>*/ [];
     for (var node in nodes) result.add(_visit(node) as dynamic/*=T*/);
     return result;
   }
 
   /// Visits a list of expressions, creating a comma expression if needed in JS.
   JS.Expression _visitListToBinary(List<Expression> nodes, String operator) {
     if (nodes == null || nodes.isEmpty) return null;
     return new JS.Expression.binary(
@@ skipping 389 matching lines @@
   var targetIdentifier = target as SimpleIdentifier;
 
   if (targetIdentifier.staticElement is! PrefixElement) return false;
   var prefix = targetIdentifier.staticElement as PrefixElement;
 
   // The library the prefix is referring to must come from a deferred import.
   var containingLibrary = (target.root as CompilationUnit).element.library;
   var imports = containingLibrary.getImportsWithPrefix(prefix);
   return imports.length == 1 && imports[0].isDeferred;
 }