Fix canonicalization of --url_mapping URIs in gen_snapshot

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';
@@ skipping 673 matching lines @@
   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();
@@ skipping 149 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;
 }