Filter unreachable libraries in kernel loader

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 6bea483442eeb0adb0de94c71a80283b64202369..9ad77ee6ef0b151fa79acc471e221621d1714c7a 100644
--- a/pkg/compiler/lib/src/library_loader.dart
+++ b/pkg/compiler/lib/src/library_loader.dart
@@ -874,13 +874,49 @@ class KernelLibraryLoaderTask extends CompilerTask
   // Only visible for unit testing.
   LoadedLibraries createLoadedLibraries(ir.Program program) {
     _elementMap.addProgram(program);
-    program.libraries.forEach((ir.Library library) =>
-        _allLoadedLibraries.add(_elementMap.lookupLibrary(library.importUri)));
     LibraryEntity rootLibrary = null;
+    Iterable<ir.Library> libraries = program.libraries;
     if (program.mainMethod != null) {
-      rootLibrary = _elementMap
-          .lookupLibrary(program.mainMethod.enclosingLibrary.importUri);
+      var root = program.mainMethod.enclosingLibrary;
+      rootLibrary = _elementMap.lookupLibrary(root.importUri);
+
+      // Filter unreachable libraries: [Program] was built by linking in the
+      // entire SDK libraries, not all of them are used. We include anything
+      // that is reachable from `main` and a few libraries that the compiler
+      // relies on.
+      var seen = new Set<Library>();
+      search(ir.Library current) {
+        if (!seen.add(current)) return;
+        for (ir.LibraryDependency dep in current.dependencies) {
+          search(dep.targetLibrary);
+        }
+      }
+
+      search(root);
+
+      const requiredLibraries = const [
+        '_foreign_helper',
+        '_interceptors',
+        '_internal',
+        '_js_embedded_names',
+        '_js_helpers',
+        '_native_typed_data',

[sra1, 2017/08/03 22:26:13]

This will be a problem for the dart2js_native tests since it contains @Native
annotations.
Can the be pulled in transitively from the patched dart:typed_data?

+        'async',
+        'core',
+      ];
+
+      for (var lib in libraries) {
+        var uri = lib.importUri;
+        if (uri.scheme != 'dart') continue;
+        if (requiredLibraries.contains(uri.path)) {
+          search(lib);
+        }
+      }
+
+      libraries = libraries.where(seen.contains);
     }
+    _allLoadedLibraries.addAll(
+        libraries.map((lib) => _elementMap.lookupLibrary(lib.importUri)));
     return new _LoadedLibrariesAdapter(
         rootLibrary, _allLoadedLibraries, _elementMap);
   }