Add a way to use shared CanonicalName root to deserialize Program.

pkg/kernel/lib/ast.dart

Index: pkg/kernel/lib/ast.dart
diff --git a/pkg/kernel/lib/ast.dart b/pkg/kernel/lib/ast.dart
index f051b14ade22132f252350ab363b45c0099bf252..bbb714990cef81b28a087ae543e7f8b1e69c64b1 100644
--- a/pkg/kernel/lib/ast.dart
+++ b/pkg/kernel/lib/ast.dart
@@ -4216,7 +4216,7 @@ class Supertype extends Node {
 
 /// A way to bundle up all the libraries in a program.
 class Program extends TreeNode {
-  final CanonicalName root = new CanonicalName.root();
+  final CanonicalName root;

[Siggi Cherem (dart-lang), 2017/05/10 18:00:38]

Question for Kevin on API design here:
 - should we have a notion of a "Bundle" or something other than Program here
for our use-case?
 - if we do, we might still need a mechanism to create a program from a bundle
and avoid recomputing the canonical names, for that purpose alone it might be
valuable to allow passing in an existing root canonical name.

[Kevin Millikin (Google), 2017/05/11 08:22:55]

Even without these changes, Program is already not quite the right name.  The
name is a left-over from when we had a distinction between the binary format for
a (linked) program and (unlinked) bundle of libraries.

Let's just keep it for now.

 
   final List<Library> libraries;
 
@@ -4228,14 +4228,19 @@ class Program extends TreeNode {
   /// Reference to the main method in one of the libraries.
   Reference mainMethodName;
 
-  Program([List<Library> libraries, Map<String, Source> uriToSource])
-      : libraries = libraries ?? <Library>[],
+  Program(
+      {CanonicalName nameRoot,
+      List<Library> libraries,
+      Map<String, Source> uriToSource})
+      : root = nameRoot ?? new CanonicalName.root(),
+        libraries = libraries ?? <Library>[],
         uriToSource = uriToSource ?? <String, Source>{} {
     setParents(this.libraries, this);
   }
 
   void computeCanonicalNames() {
     for (var library in libraries) {
+      if (library.canonicalName != null) continue;

[Kevin Millikin (Google), 2017/05/11 08:22:55]

Keep in mind that this approach of skipping libraries only works if a library
having a canonical name implies that every (named) thing in that library also
has a canonical name.

I don't know enough about your use case to know if that's true, but I'd rather
not assume it in package Kernel if we can think of a way around it.

(I'm thinking that you might want to read an external library from a binary, in
which case the library will have a canonical name.  Then later you might read
some named member of the library from some other source than a Kernel binary, in
which case it will not have a canonical name.  Skipping the library will never
build the name for that member.)

I'm also concerned about the converse problem.  You could imagine a Kernel
transformation that introduced fresh libraries (without a canonical name
computed) built out of members that were deserialized and so had a canonical
name.  For instance, maybe you wanted a transformation that combined distinct
libraries into new ones.  In that case, you will start naming the library and
then name the children and still hit the assert that you don't name something
that already has a name.

[scheglov, 2017/05/11 18:31:25]

OK, I rolled this back.

       root.getChildFromUri(library.importUri).bindTo(library.reference);
       library.computeCanonicalNames();
     }