Support serialization of the compiler backbone.

pkg/compiler/lib/src/library_loader.dart

Index: pkg/compiler/lib/src/library_loader.dart
diff --git a/pkg/compiler/lib/src/library_loader.dart b/pkg/compiler/lib/src/library_loader.dart
index a566d3f27d33b24bb5cb2e9152311f66648137da..172435d093553cc0d7b347c725e9a7b043f234af 100644
--- a/pkg/compiler/lib/src/library_loader.dart
+++ b/pkg/compiler/lib/src/library_loader.dart
@@ -482,7 +482,7 @@ class _LibraryLoaderTask extends CompilerTask implements LibraryLoaderTask {
           then((Script sourceScript) {
             if (sourceScript == null) return;
 
-            CompilationUnitElement unit =
+            CompilationUnitElementX unit =
                 new CompilationUnitElementX(sourceScript, library);
             compiler.withCurrentElement(unit, () {
               compiler.scanner.scan(unit);
@@ -523,6 +523,19 @@ class _LibraryLoaderTask extends CompilerTask implements LibraryLoaderTask {
         });
   }
 
+  Future<LibraryElement> readLibrary(LibraryDependencyHandler handler,
+                                     Uri resolvedUri,
+                                     LibraryElement library) async {
+    compiler.onLibraryCreated(library);
+    libraryCanonicalUriMap[resolvedUri] = library;
+    await compiler.onLibraryScanned(library, handler);
+    for (LibraryTag tag in library.tags) {
+      LibraryElement dependency = library.getLibraryFromTag(tag);
+      await createLibrary(handler, library, dependency.canonicalUri);
+    }
+    return library;
+  }
+
   /**
    * Create (or reuse) a library element for the library specified by the
    * [resolvedUri].
@@ -539,6 +552,10 @@ class _LibraryLoaderTask extends CompilerTask implements LibraryLoaderTask {
     if (library != null) {
       return new Future.value(library);
     }
+    library = compiler.serialization.readLibrary(resolvedUri);

[floitsch, 2015/06/26 20:54:04]

This is confusing:
we have two readLibrary, but they don't do the same thing.
It seems like the second "readLibrary" doesn't have a good name.

[Johnni Winther, 2015/07/03 12:13:44]

Done.

+    if (library != null) {
+      return readLibrary(handler, resolvedUri, library);
+    }
     var readScript = compiler.readScript;
     if (readableUri == null) {
       readableUri = resolvedUri;