Implement library access restrictions and privacy.

pkg/front_end/lib/src/fasta/builder/library_builder.dart

 // Copyright (c) 2016, 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 fasta.library_builder;
 
 import '../combinator.dart' show Combinator;
 
 import '../errors.dart' show InputError, internalError, printUnexpected;
 
@@ skipping 114 matching lines @@
   /// If [constructorName] is null or the empty string, it's assumed to be an
   /// unnamed constructor.
   Builder getConstructor(String className,
       {String constructorName, bool isPrivate: false}) {
     constructorName ??= "";
     Builder cls = (isPrivate ? scope : exports).lookup(className, -1, null);
     if (cls is ClassBuilder) {
       // TODO(ahe): This code is similar to code in `endNewExpression` in
       // `body_builder.dart`, try to share it.
       Builder constructor =
-          cls.findConstructorOrFactory(constructorName, -1, null);
+          cls.findConstructorOrFactory(constructorName, -1, null, this);

[Paul Berry, 2017/06/01 16:54:18]

This method (getConstructor) is written in a really general purpose fashion but
it's only used for a few very specific purposes (looking up the unnamed
constructors for 'AbstractClassInstantiationError', '_CompileTimeError',
'ExternalName', 'FallThroughError', and 'NoSuchMethodError').  For these
specific use cases, passing `this` to findConstructorOrFactory is safe.

But I'm concerned that if someone tries to make more general purpose use of this
method in the future it will mask bugs.

Can we make one of the following changes?

1. Get rid of the [constructorName] argument (since no current caller uses it). 
That way, passing `this` is safe.  Or:

2. Add a required `accessingLibrary` argument to `getConstructor`, so that
callers are required to say where they are trying to access the constructor
from.

[ahe, 2017/06/01 17:12:42]

Good point.
The constructorName will be used soon.
That's kinda what the `isPrivate` parameter is for. Here's what I'm thinking:

Rename parameter `isPrivate` parameter to `bypassLibraryPrivacy`.

If `constructorName` starts with "_", `bypassLibraryPrivacy` must be true.

[Paul Berry, 2017/06/01 17:15:11]

sgtm, thanks.

[ahe, 2017/06/02 06:26:51]

Done.

       if (constructor == null) {
         // Fall-through to internal error below.
       } else if (constructor.isConstructor) {
         if (!cls.isAbstract) {
           return constructor;
         }
       } else if (constructor.isFactory) {
         return constructor;
       }
     }
@@ skipping 16 matching lines @@
   /// Don't use for scope lookup. Only use when an element is known to exist
   /// (and not a setter).
   Builder operator [](String name) {
     return scope.local[name] ?? internalError("Not found: '$name'.");
   }
 
   Builder lookup(String name, int charOffset, Uri fileUri) {
     return scope.lookup(name, charOffset, fileUri);
   }
 }