Rework standalone to use a synchronous loader that does not invoke Dart code

runtime/bin/builtin.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 builtin;
 // NOTE: Do not import 'dart:io' in builtin.
 import 'dart:async';
 import 'dart:collection';
 import 'dart:_internal';
 import 'dart:isolate';
 import 'dart:typed_data';
 
+// Embedder sets this to true if the --trace-loading flag was passed on the
+// command line.
+bool _traceLoading = false;
+
 
 // Before handling an embedder entrypoint we finalize the setup of the
 // dart:_builtin library.
 bool _setupCompleted = false;
 
 
 // The root library (aka the script) is imported into this library. The
 // standalone embedder uses this to lookup the main entrypoint in the
 // root library's namespace.
 Function _getMainClosure() => main;
@@ skipping 22 matching lines @@
   // We are not using Dircetory.current here to limit the dependency
   // on dart:io. This code is the same as:
   //   return new Uri.directory(Directory.current.path);
   var path = _getCurrentDirectoryPath();
   return new Uri.directory(path);
 }
 
 
 _getUriBaseClosure() => _uriBase;
 
-
 // Asynchronous loading of resources.
-// The embedder forwards most loading requests to this library.
-
-// See Dart_LibraryTag in dart_api.h
-const _Dart_kScriptTag = null;
-const _Dart_kImportTag = 0;
-const _Dart_kSourceTag = 1;
-const _Dart_kCanonicalizeUrl = 2;
-const _Dart_kResourceLoad = 3;
-
-// Embedder sets this to true if the --trace-loading flag was passed on the
-// command line.
-bool _traceLoading = false;
-
-// This is currently a build time flag only. We measure the time from the first
-// load request (opening the receive port) to completing the last load
-// request (closing the receive port). Future, deferred load operations will
-// add to this time.
-bool _timeLoading = false;
-Stopwatch _stopwatch;
+// The embedder forwards loading requests to the service isolate.
 
 // A port for communicating with the service isolate for I/O.
 SendPort _loadPort;
-// The receive port for a load request. Multiple sources can be fetched in
-// a single load request.
-RawReceivePort _dataPort;
-// A request id valid only for the current load cycle (while the number of
-// outstanding load requests is greater than 0). Can be reset when loading is
-// completed.
-int _reqId = 0;
-// An unordered hash map mapping from request id to a particular load request.
-// Once there are no outstanding load requests the current load has finished.
-HashMap _reqMap = new HashMap();
+
+// The isolateId used to communicate with the service isolate for I/O.
+int _isolateId;
+
+// Requests made to the service isolate over the load port.
+
+// Extra requests. Keep these in sync between loader.dart and builtin.dart.
+const _Dart_kInitLoader = 4;           // Initialize the loader.
+const _Dart_kResourceLoad = 5;         // Resource class support.
+const _Dart_kGetPackageRootUri = 6;    // Uri of the packages/ directory.
+const _Dart_kGetPackageConfigUri = 7;  // Uri of the .packages file.
+const _Dart_kResolvePackageUri = 8;    // Resolve a package: uri.
+
+// Make a request to the loader. Future will complete with result which is
+// either a Uri or a List<int>.
+Future _makeLoaderRequest(int tag, String uri) {
+  assert(_isolateId != null);
+  assert(_loadPort != null);
+  Completer completer = new Completer();
+  RawReceivePort port = new RawReceivePort();
+  port.handler = (msg) {
+    // Close the port.
+    port.close();
+    completer.complete(msg);
+  };
+  _loadPort.send([_traceLoading, _isolateId, tag, port.sendPort, uri]);
+  return completer.future;
+}
+
 
 // The current working directory when the embedder was launched.
 Uri _workingDirectory;
 // The URI that the root script was loaded from. Remembered so that
 // package imports can be resolved relative to it. The root script is the basis
 // for the root library in the VM.
 Uri _rootScript;
-
-// Packages are either resolved looking up in a map or resolved from within a
-// package root.
-bool get _packagesReady =>
-    (_packageRoot != null) || (_packageMap != null) || (_packageError != null);
-// Error string set if there was an error resolving package configuration.
-// For example not finding a .packages file or packages/ directory, malformed
-// .packages file or any other related error.
-String _packageError = null;
-// The directory to look in to resolve "package:" scheme URIs. By detault it is
-// the 'packages' directory right next to the script.
-Uri _packageRoot = null; // Used to be _rootScript.resolve('packages/');
-// The map describing how certain package names are mapped to Uris.
-Uri _packageConfig = null;
-Map<String, Uri> _packageMap = null;
-
-// A list of pending packags which have been requested while resolving the
-// location of the package root or the contents of the package map.
-List<_LoadRequest> _pendingPackageLoads = [];
-
-// If we have outstanding loads or pending package loads waiting for resolution,
-// then we do have pending loads.
-bool _pendingLoads() => !_reqMap.isEmpty || !_pendingPackageLoads.isEmpty;
+// The package root set on the command line.
+Uri _packageRoot;
 
 // Special handling for Windows paths so that they are compatible with URI
 // handling.
 // Embedder sets this to true if we are running on Windows.
 bool _isWindows = false;
 
 // Logging from builtin.dart is prefixed with a '*'.
 String _logId = (Isolate.current.hashCode % 0x100000).toRadixString(16);
 _log(msg) {
   _print("* $_logId $msg");
 }
 
-// A class wrapping the load error message in an Error object.
-class _LoadError extends Error {
-  final _LoadRequest request;
-  final String message;
-  _LoadError(this.request, this.message);
-
-  String toString() {
-    var context = request._context;
-    if (context == null || context is! String) {
-      return 'Could not load "${request._uri}": $message';
-    } else {
-      return 'Could not import "${request._uri}" from "$context": $message';
-    }
-  }
-}
-
-// Class collecting all of the information about a particular load request.
-class _LoadRequest {
-  final int _id = _reqId++;
-  final int _tag;
-  final String _uri;
-  final Uri _resourceUri;
-  final _context;
-
-  _LoadRequest(this._tag, this._uri, this._resourceUri, this._context) {
-    assert(_reqMap[_id] == null);
-    _reqMap[_id] = this;
-  }
-
-  toString() => "LoadRequest($_id, $_tag, $_uri, $_resourceUri, $_context)";
-}
-
-
-// Native calls provided by the embedder.
-void _signalDoneLoading() native "Builtin_DoneLoading";
-void _loadScriptCallback(int tag, String uri, String libraryUri, Uint8List data)
-    native "Builtin_LoadSource";
-void _asyncLoadErrorCallback(uri, libraryUri, error)
-    native "Builtin_AsyncLoadError";
-
-
 _sanitizeWindowsPath(path) {
   // For Windows we need to massage the paths a bit according to
   // http://blogs.msdn.com/b/ie/archive/2006/12/06/file-uris-in-windows.aspx
   //
   // Convert
   // C:\one\two\three
   // to
   // /C:/one/two/three
 
   if (_isWindows == false) {
@@ skipping 31 matching lines @@
 
 
 // Ensure we have a trailing slash character.
 _enforceTrailingSlash(uri) {
   if (!uri.endsWith('/')) {
     return '$uri/';
   }
   return uri;
 }
 
-
 // Embedder Entrypoint:
 // The embedder calls this method with the current working directory.
 void _setWorkingDirectory(cwd) {
   if (!_setupCompleted) {
     _setupHooks();
   }
   if (_traceLoading) {
     _log('Setting working directory: $cwd');
   }
   _workingDirectory = new Uri.directory(cwd);
@@ skipping 23 matching lines @@
     _packageRoot = _workingDirectory.resolveUri(new Uri.file(packageRoot));
   }
   // Now that we have determined the packageRoot value being used, set it
   // up for use in Platform.packageRoot. This is only set when the embedder
   // sets up the package root. Automatically discovered package root will
   // not update the VMLibraryHooks value.
   VMLibraryHooks.packageRootString = _packageRoot.toString();
   if (_traceLoading) {
     _log('Package root URI: $_packageRoot');
   }
-}
-
-
-// Given a uri with a 'package' scheme, return a Uri that is prefixed with
-// the package root.
-Uri _resolvePackageUri(Uri uri) {
-  assert(uri.scheme == "package");
-  assert(_packagesReady);
-
-  if (!uri.host.isEmpty) {
-    var path = '${uri.host}${uri.path}';
-    var right = 'package:$path';
-    var wrong = 'package://$path';
-
-    throw "URIs using the 'package:' scheme should look like "
-          "'$right', not '$wrong'.";
-  }
-
-  if (_traceLoading) {
-    _log('Resolving package with uri path: ${uri.path}');
-  }
-  var resolvedUri;
-  if (_packageError != null) {
-    if (_traceLoading) {
-      _log("Resolving package with pending resolution error: $_packageError");
-    }
-    throw _packageError;
-  } else if (_packageRoot != null) {
-    resolvedUri = _packageRoot.resolve(uri.path);
-  } else {
-    var packageName = uri.pathSegments[0];
-    var mapping = _packageMap[packageName];
-    if (_traceLoading) {
-      _log("Mapped '$packageName' package to '$mapping'");
-    }
-    if (mapping == null) {
-      throw "No mapping for '$packageName' package when resolving '$uri'.";
-    }
-    var path;
-    if (uri.path.length > packageName.length) {
-      path = uri.path.substring(packageName.length + 1);
-    } else {
-      // Handle naked package resolution to the default package name:
-      // package:foo is equivalent to package:foo/foo.dart
-      assert(uri.path.length == packageName.length);
-      path = "$packageName.dart";
-    }
-    if (_traceLoading) {
-      _log("Path to be resolved in package: $path");
-    }
-    resolvedUri = mapping.resolve(path);
-  }
-  if (_traceLoading) {
-    _log("Resolved '$uri' to '$resolvedUri'.");
-  }
-  return resolvedUri;
-}
-
-
-// Resolves the script uri in the current working directory iff the given uri
-// did not specify a scheme (e.g. a path to a script file on the command line).
-Uri _resolveScriptUri(String scriptName) {
-  if (_traceLoading) {
-    _log("Resolving script: $scriptName");
-  }
-  if (_workingDirectory == null) {
-    throw 'No current working directory set.';
-  }
-  scriptName = _sanitizeWindowsPath(scriptName);
-
-  var scriptUri = Uri.parse(scriptName);
-  if (scriptUri.scheme == '') {
-    // Script does not have a scheme, assume that it is a path,
-    // resolve it against the working directory.
-    scriptUri = _workingDirectory.resolveUri(scriptUri);
-  }
-
-  // Remember the root script URI so that we can resolve packages based on
-  // this location.
-  _rootScript = scriptUri;
-
-  if (_traceLoading) {
-    _log('Resolved entry point to: $_rootScript');
-  }
-  return scriptUri;
-}
-
-
-void _finishLoadRequest(_LoadRequest req) {
-  if (req != null) {
-    // Now that we are done with loading remove the request from the map.
-    var tmp = _reqMap.remove(req._id);
-    assert(tmp == req);
-    if (_traceLoading) {
-      _log("Loading of ${req._uri} finished: "
-           "${_reqMap.length} requests remaining, "
-           "${_pendingPackageLoads.length} packages pending.");
-    }
-  }
-
-  if (!_pendingLoads() && (_dataPort != null)) {
-    _stopwatch.stop();
-    // Close the _dataPort now that there are no more requests outstanding.
-    if (_traceLoading || _timeLoading) {
-      _log("Closing loading port: ${_stopwatch.elapsedMilliseconds} ms");
-    }
-    _dataPort.close();
-    _dataPort = null;
-    _reqId = 0;
-    _signalDoneLoading();
-  }
-}
-
-
-void _handleLoaderReply(msg) {
-  int id = msg[0];
-  var dataOrError = msg[1];
-  assert((id >= 0) && (id < _reqId));
-  var req = _reqMap[id];
-  try {
-    if (dataOrError is Uint8List) {
-      // Successfully loaded the data.
-      if (req._tag == _Dart_kResourceLoad) {
-        Completer c = req._context;
-        c.complete(dataOrError);
-      } else {
-        // TODO: Currently a compilation error while loading the script is
-        // fatal for the isolate. _loadScriptCallback() does not return and
-        // the number of requests remains out of sync.
-        _loadScriptCallback(req._tag, req._uri, req._context, dataOrError);
-      }
-      _finishLoadRequest(req);
-    } else {
-      assert(dataOrError is String);
-      var error = new _LoadError(req, dataOrError.toString());
-      _asyncLoadError(req, error, null);
-    }
-  } catch(e, s) {
-    // Wrap inside a _LoadError unless we are already propagating a
-    // previous _LoadError.
-    var error = (e is _LoadError) ? e : new _LoadError(req, e.toString());
-    assert(req != null);
-    _asyncLoadError(req, error, s);
-  }
-}
-
-
-void _startLoadRequest(int tag, String uri, Uri resourceUri, context) {
-  if (_dataPort == null) {
-    if (_traceLoading) {
-      _log("Initializing load port.");
-    }
-    // Allocate the Stopwatch if necessary.
-    if (_stopwatch == null) {
-      _stopwatch = new Stopwatch();
-    }
-    assert(_dataPort == null);
-    _dataPort = new RawReceivePort(_handleLoaderReply);
-    _stopwatch.start();
-  }
-  // Register the load request and send it to the VM service isolate.
-  var req = new _LoadRequest(tag, uri, resourceUri, context);
-
-  assert(_dataPort != null);
-  var msg = new List(4);
-  msg[0] = _dataPort.sendPort;
-  msg[1] = _traceLoading;
-  msg[2] = req._id;
-  msg[3] = resourceUri.toString();
-  _loadPort.send(msg);
-
-  if (_traceLoading) {
-    _log("Loading of $resourceUri for $uri started with id: ${req._id}. "
-         "${_reqMap.length} requests remaining, "
-         "${_pendingPackageLoads.length} packages pending.");
-  }
-}
-
-
-RawReceivePort _packagesPort;
-
-void _handlePackagesReply(msg) {
-  // Make sure to close the _packagePort before any other action.
-  _packagesPort.close();
-  _packagesPort = null;
-
-  if (_traceLoading) {
-    _log("Got packages reply: $msg");
-  }
-  if (msg is String) {
-    if (_traceLoading) {
-      _log("Got failure response on package port: '$msg'");
-    }
-    // Remember the error message.
-    _packageError = msg;
-  } else if (msg is List) {
-    if (msg.length == 1) {
-      if (_traceLoading) {
-        _log("Received package root: '${msg[0]}'");
-      }
-      _packageRoot = Uri.parse(msg[0]);
-    } else {
-      // First entry contains the location of the loaded .packages file.
-      assert((msg.length % 2) == 0);
-      assert(msg.length >= 2);
-      assert(msg[1] == null);
-      _packageConfig = Uri.parse(msg[0]);
-      _packageMap = new Map<String, Uri>();
-      for (var i = 2; i < msg.length; i+=2) {
-        // TODO(iposva): Complain about duplicate entries.
-        _packageMap[msg[i]] = Uri.parse(msg[i+1]);
-      }
-      if (_traceLoading) {
-        _log("Setup package map: $_packageMap");
-      }
-    }
-  } else {
-    _packageError = "Bad type of packages reply: ${msg.runtimeType}";
-    if (_traceLoading) {
-      _log(_packageError);
-    }
-  }
-
-  // Resolve all pending package loads now that we know how to resolve them.
-  while (_pendingPackageLoads.length > 0) {
-    // Order does not matter as we queue all of the requests up right now.
-    var req = _pendingPackageLoads.removeLast();
-    // Call the registered closure, to handle the delayed action.
-    req();
-  }
-  // Reset the pending package loads to empty. So that we eventually can
-  // finish loading.
-  _pendingPackageLoads = [];
-  // Make sure that the receive port is closed if no other loads are pending.
-  _finishLoadRequest(null);
-}
-
-
-void _requestPackagesMap() {
-  assert(_packagesPort == null);
-  assert(_rootScript != null);
-  // Create a port to receive the packages map on.
-  _packagesPort = new RawReceivePort(_handlePackagesReply);
-  var sp = _packagesPort.sendPort;
-
-  var msg = new List(4);
-  msg[0] = sp;
-  msg[1] = _traceLoading;
-  msg[2] = -1;
-  msg[3] = _rootScript.toString();
-  _loadPort.send(msg);
-
-  if (_traceLoading) {
-    _log("Requested packages map for '$_rootScript'.");
-  }
+  return VMLibraryHooks.packageRootString;

[turnidge, 2016/06/03 17:50:24]

Is this return value used?

[Cutch, 2016/06/03 22:07:32]

Removed this. 

 }
 
 
 // Embedder Entrypoint:
-// Request the load of a particular packages map.
-void _loadPackagesMap(String packagesParam) {
+void _setPackagesMap(String packagesParam) {
   if (!_setupCompleted) {
     _setupHooks();
   }
   // First convert the packages parameter from the command line to a URI which
   // can be handled by the loader code.
   // TODO(iposva): Consider refactoring the common code below which is almost
   // shared with resolution of the root script.
   if (_traceLoading) {
     _log("Resolving packages map: $packagesParam");
   }
   if (_workingDirectory == null) {
     throw 'No current working directory set.';
   }
   var packagesName = _sanitizeWindowsPath(packagesParam);
   var packagesUri = Uri.parse(packagesName);
   if (packagesUri.scheme == '') {
     // Script does not have a scheme, assume that it is a path,
     // resolve it against the working directory.
     packagesUri = _workingDirectory.resolveUri(packagesUri);
   }
   var packagesUriStr = packagesUri.toString();
   VMLibraryHooks.packageConfigString = packagesUriStr;
   if (_traceLoading) {
     _log('Resolved packages map to: $packagesUri');
   }
-
-  // Request the loading and parsing of the packages map at the specified URI.
-  // Create a port to receive the packages map on.
-  assert(_packagesPort == null);
-  _packagesPort = new RawReceivePort(_handlePackagesReply);
-  var sp = _packagesPort.sendPort;
-
-  var msg = new List(4);
-  msg[0] = sp;
-  msg[1] = _traceLoading;
-  msg[2] = -2;
-  msg[3] = packagesUriStr;
-  _loadPort.send(msg);
-
-  // Signal that the resolution of the packages map has started. But in this
-  // case it is not tied to a particular request.
-  _pendingPackageLoads.add(() {
-    // Nothing to be done beyond registering that there is pending package
-    // resolution requested by having an empty entry.
-    if (_traceLoading) {
-      _log("Skipping dummy deferred request.");
-    }
-  });
-
-  if (_traceLoading) {
-    _log("Requested packages map at '$packagesUri'.");
-  }
 }
 
 
-void _asyncLoadError(_LoadRequest req, _LoadError error, StackTrace stack) {
+// Resolves the script uri in the current working directory iff the given uri
+// did not specify a scheme (e.g. a path to a script file on the command line).
+String _resolveScriptUri(String scriptName) {
   if (_traceLoading) {
-    _log("_asyncLoadError(${req._uri}), error: $error\nstack: $stack");
+    _log("Resolving script: $scriptName");
   }
-  if (req._tag == _Dart_kResourceLoad) {
-    Completer c = req._context;
-    c.completeError(error, stack);
-  } else {
-    String libraryUri = req._context;
-    if (req._tag == _Dart_kImportTag) {
-      // When importing a library, the libraryUri is the imported
-      // uri.
-      libraryUri = req._uri;
-    }
-    _asyncLoadErrorCallback(req._uri, libraryUri, error);
+  if (_workingDirectory == null) {
+    throw 'No current working directory set.';
   }
-  _finishLoadRequest(req);
+  scriptName = _sanitizeWindowsPath(scriptName);
+
+  var scriptUri = Uri.parse(scriptName);
+  if (scriptUri.scheme == '') {
+    // Script does not have a scheme, assume that it is a path,
+    // resolve it against the working directory.
+    scriptUri = _workingDirectory.resolveUri(scriptUri);
+  }
+
+  // Remember the root script URI so that we can resolve packages based on
+  // this location.
+  _rootScript = scriptUri;
+
+  if (_traceLoading) {
+    _log('Resolved entry point to: $_rootScript');
+  }
+  return scriptUri.toString();
 }
 
 
-_loadDataFromLoadPort(int tag, String uri, Uri resourceUri, context) {
-  try {
-    _startLoadRequest(tag, uri, resourceUri, context);
-  } catch (e, s) {
-    if (_traceLoading) {
-      _log("Exception when communicating with service isolate: $e");
-    }
-    // Register a dummy load request so we can fail to load it.
-    var req = new _LoadRequest(tag, uri, resourceUri, context);
-
-    // Wrap inside a _LoadError unless we are already propagating a previously
-    // seen _LoadError.
-    var error = (e is _LoadError) ? e : new _LoadError(req, e.toString());
-    _asyncLoadError(req, error, s);
-  }
-}
-
-
-// Loading a package URI needs to first map the package name to a loadable
-// URI.
-_loadPackage(int tag, String uri, Uri resourceUri, context) {
-  if (_packagesReady) {
-    var resolvedUri;
-    try {
-      resolvedUri = _resolvePackageUri(resourceUri);
-    } catch (e, s) {
-      if (_traceLoading) {
-        _log("Exception ($e) when resolving package URI: $resourceUri");
-      }
-      // Register a dummy load request so we can fail to load it.
-      var req = new _LoadRequest(tag, uri, resourceUri, context);
-
-      // Wrap inside a _LoadError unless we are already propagating a previously
-      // seen _LoadError.
-      var error = (e is _LoadError) ? e : new _LoadError(req, e.toString());
-      _asyncLoadError(req, error, s);
-    }
-    _loadData(tag, uri, resolvedUri, context);
-  } else {
-    if (_pendingPackageLoads.isEmpty) {
-      // Package resolution has not been setup yet, and this is the first
-      // request for package resolution & loading.
-      _requestPackagesMap();
-    }
-    // Register the action of loading this package once the package resolution
-    // is ready.
-    _pendingPackageLoads.add(() {
-      if (_traceLoading) {
-        _log("Handling deferred package request: "
-             "$tag, $uri, $resourceUri, $context");
-      }
-      _loadPackage(tag, uri, resourceUri, context);
-    });
-    if (_traceLoading) {
-      _log("Pending package load of '$uri': "
-           "${_pendingPackageLoads.length} pending");
-    }
-  }
-}
-
-
-// Load the data associated with the resourceUri.
-_loadData(int tag, String uri, Uri resourceUri, context) {
-  if (resourceUri.scheme == 'package') {
-    // package based uris need to be resolved to the correct loadable location.
-    // The logic of which is handled seperately, and then _loadData is called
-    // recursively.
-    _loadPackage(tag, uri, resourceUri, context);
-  } else {
-    _loadDataFromLoadPort(tag, uri, resourceUri, context);
-  }
-}
-
-
-// Embedder Entrypoint:
-// Asynchronously loads script data through a http[s] or file uri.
-_loadDataAsync(int tag, String uri, String libraryUri) {
-  if (!_setupCompleted) {
-    _setupHooks();
-  }
-  var resourceUri;
-  if (tag == _Dart_kScriptTag) {
-    resourceUri = _resolveScriptUri(uri);
-    uri = resourceUri.toString();
-  } else {
-    resourceUri = Uri.parse(uri);
-  }
-  _loadData(tag, uri, resourceUri, libraryUri);
-}
-
-
 // Embedder Entrypoint:
 // Function called by standalone embedder to resolve uris when the VM requests
 // Dart_kCanonicalizeUrl from the tag handler.
 String _resolveUri(String base, String userString) {
   if (!_setupCompleted) {
     _setupHooks();
   }
   if (_traceLoading) {
     _log('Resolving: $userString from $base');
   }
   var baseUri = Uri.parse(base);
   var result = baseUri.resolve(userString).toString();
   if (_traceLoading) {
     _log('Resolved $userString in $base to $result');
   }
   return result;
 }
 
 
-// Handling of access to the package root or package map from user code.
-_triggerPackageResolution(action) {
-  if (_packagesReady) {
-    // Packages are ready. Execute the action now.
-    action();
-  } else {
-    if (_pendingPackageLoads.isEmpty) {
-      // Package resolution has not been setup yet, and this is the first
-      // request for package resolution & loading.
-      _requestPackagesMap();
-    }
-    // Register the action for when the package resolution is ready.
-    _pendingPackageLoads.add(action);
-  }
-}
-
-
-Future<Uri> _getPackageRootFuture() {
-  if (_traceLoading) {
-    _log("Request for package root from user code.");
-  }
-  var completer = new Completer<Uri>();
-  _triggerPackageResolution(() {
-    completer.complete(_packageRoot);
-  });
-  return completer.future;
-}
-
-
-Future<Uri> _getPackageConfigFuture() {
-  if (_traceLoading) {
-    _log("Request for package config from user code.");
-  }
-  var completer = new Completer<Uri>();
-  _triggerPackageResolution(() {
-    completer.complete(_packageConfig);
-  });
-  return completer.future;
-}
-
-
-Future<Uri> _resolvePackageUriFuture(Uri packageUri) async {
-  if (_traceLoading) {
-    _log("Request for package Uri resolution from user code: $packageUri");
-  }
-  if (packageUri.scheme != "package") {
-    if (_traceLoading) {
-      _log("Non-package Uri, returning unmodified: $packageUri");
-    }
-    // Return the incoming parameter if not passed a package: URI.
-    return packageUri;
-  }
-
-  if (!_packagesReady) {
-    if (_traceLoading) {
-      _log("Trigger loading by requesting the package config.");
-    }
-    // Make sure to trigger package resolution.
-    var dummy = await _getPackageConfigFuture();
-  }
-  assert(_packagesReady);
-
-  var result;
-  try {
-    result = _resolvePackageUri(packageUri);
-  } catch (e, s) {
-    // Any error during resolution will resolve this package as not mapped,
-    // which is indicated by a null return.
-    if (_traceLoading) {
-      _log("Exception ($e) when resolving package URI: $packageUri");
-    }
-    result = null;
-  }
-  if (_traceLoading) {
-    _log("Resolved '$packageUri' to '$result'");
-  }
-  return result;
-}
-
-
-// Handling of Resource class by dispatching to the load port.
-Future<List<int>> _resourceReadAsBytes(Uri uri) {
-  var completer = new Completer<List<int>>();
-  // Request the load of the resource associating the completer as the context
-  // for the load.
-  _loadData(_Dart_kResourceLoad, uri.toString(), uri, completer);
-  // Return the future that will be triggered once the resource has been loaded.
-  return completer.future;
-}
-
-
 // Embedder Entrypoint (gen_snapshot):
 // Resolve relative paths relative to working directory.
 String _resolveInWorkingDirectory(String fileName) {
   if (!_setupCompleted) {
     _setupHooks();
   }
   if (_workingDirectory == null) {
     throw 'No current working directory set.';
   }
   var name = _sanitizeWindowsPath(fileName);
 
   var uri = Uri.parse(name);
   if (uri.scheme != '') {
     throw 'Schemes are not supported when resolving filenames.';
   }
   uri = _workingDirectory.resolveUri(uri);
 
   if (_traceLoading) {
     _log('Resolved in working directory: $fileName -> $uri');
   }
   return uri.toString();
 }
 
+// Only used by vm/cc unit tests.
+Uri _resolvePackageUri(Uri uri) {
+  assert(_packageRoot != null);
+  return _packageRoot.resolve(uri.path);
+}
+
 
 // Returns either a file path or a URI starting with http[s]:, as a String.
 String _filePathFromUri(String userUri) {
   var uri = Uri.parse(userUri);
   if (_traceLoading) {
     _log('Getting file path from: $uri');
   }
 
   var path;
   switch (uri.scheme) {
@@ skipping 35 matching lines @@
 
 // Register callbacks and hooks with the rest of the core libraries.
 _setupHooks() {
   _setupCompleted = true;
   VMLibraryHooks.resourceReadAsBytes = _resourceReadAsBytes;
 
   VMLibraryHooks.packageRootUriFuture = _getPackageRootFuture;
   VMLibraryHooks.packageConfigUriFuture = _getPackageConfigFuture;
   VMLibraryHooks.resolvePackageUriFuture = _resolvePackageUriFuture;
 }
+
+
+// Handling of Resource class by dispatching to the load port.
+Future<List<int>> _resourceReadAsBytes(Uri uri) async {
+  List response = await _makeLoaderRequest(_Dart_kResourceLoad, uri.toString());
+  if (response[3] is String) {
+    // Throw the error.
+    throw response[3];
+  } else {
+    return response[3];
+  }
+}
+
+
+Future<Uri> _getPackageRootFuture() {
+  if (_traceLoading) {
+    _log("Request for package root from user code.");
+  }
+  return _makeLoaderRequest(_Dart_kGetPackageRootUri, null);
+}
+
+
+Future<Uri> _getPackageConfigFuture() {
+  if (_traceLoading) {
+    _log("Request for package config from user code.");
+  }
+  assert(_loadPort != null);
+  return _makeLoaderRequest(_Dart_kGetPackageConfigUri, null);
+}
+
+
+Future<Uri> _resolvePackageUriFuture(Uri packageUri) async {
+  if (_traceLoading) {
+    _log("Request for package Uri resolution from user code: $packageUri");
+  }
+  if (packageUri.scheme != "package") {
+    if (_traceLoading) {
+      _log("Non-package Uri, returning unmodified: $packageUri");
+    }
+    // Return the incoming parameter if not passed a package: URI.
+    return packageUri;
+  }
+  var result = await _makeLoaderRequest(_Dart_kResolvePackageUri,
+                                        packageUri.toString());
+  if (result is! Uri) {
+    if (_traceLoading) {
+      _log("Exception when resolving package URI: $packageUri");
+    }
+    result = null;
+  }
+  if (_traceLoading) {
+    _log("Resolved '$packageUri' to '$result'");
+  }
+  return result;
+}