Implement library access restrictions and privacy.

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

Index: pkg/front_end/lib/src/fasta/builder/library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/library_builder.dart b/pkg/front_end/lib/src/fasta/builder/library_builder.dart
index e3067aa8ccbf980457e61dbfc39343eb82734d3b..8bf7c6a22d5352cf0fc5944ac30098062d10ca85 100644
--- a/pkg/front_end/lib/src/fasta/builder/library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/library_builder.dart
@@ -132,7 +132,7 @@ abstract class LibraryBuilder<T extends TypeBuilder, R> extends Builder {
       // 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) {