Filter unreachable libraries in kernel loader

pkg/compiler/lib/src/library_loader.dart

 // Copyright (c) 2012, 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 dart2js.library_loader;
 
 import 'dart:async';
 
 import 'package:front_end/front_end.dart' as fe;
 import 'package:kernel/ast.dart' as ir;
@@ skipping 855 matching lines @@
         program = await fe.kernelForProgram(resolvedUri, options);
       }
       if (program == null) return null;
       return createLoadedLibraries(program);
     });
   }
 
   // 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`. Note that all internal libraries that
+      // the compiler relies on are reachable from `dart:core`.
+      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);
+
+      // Libraries dependencies do not show implicit imports to `dart:core`.
+      var dartCore = program.libraries.firstWhere((lib) {
+        return lib.importUri.scheme == 'dart' && lib.importUri.path == 'core';
+      });
+      search(dartCore);
+
+      libraries = libraries.where(seen.contains);
     }
+    _allLoadedLibraries.addAll(
+        libraries.map((lib) => _elementMap.lookupLibrary(lib.importUri)));
     return new _LoadedLibrariesAdapter(
         rootLibrary, _allLoadedLibraries, _elementMap);
   }
 
   KernelToElementMapForImpactImpl get elementMap => _elementMap;
 
   void reset({bool reuseLibrary(LibraryElement library)}) {
     throw new UnimplementedError('KernelLibraryLoaderTask.reset');
   }
 
@@ skipping 796 matching lines @@
 }
 
 /// API used by the library loader to synchronously scan a library or
 /// compilation unit and ensure that their library tags are computed.
 abstract class ElementScanner {
   void scanLibrary(LibraryElement library);
   void scanUnit(CompilationUnitElement unit);
 }
 
 const _reuseLibrarySubtaskName = "Reuse library";