Fix canonicalization of --url_mapping URIs in gen_snapshot

runtime/bin/dartutils.cc

Index: runtime/bin/dartutils.cc
diff --git a/runtime/bin/dartutils.cc b/runtime/bin/dartutils.cc
index 8be3b29a3ce8bfea25422945f16c5ff7c86f0542..5db7dacc0d58644d8b1fee7ee2762858795fe723 100644
--- a/runtime/bin/dartutils.cc
+++ b/runtime/bin/dartutils.cc
@@ -334,6 +334,18 @@ Dart_Handle DartUtils::SetWorkingDirectory() {
 }
 
 
+Dart_Handle DartUtils::ResolveUri(Dart_Handle library_url, Dart_Handle url) {
+  const int kNumArgs = 2;
+  Dart_Handle dart_args[kNumArgs];
+  dart_args[0] = library_url;
+  dart_args[1] = url;
+  return Dart_Invoke(DartUtils::BuiltinLib(),
+                     NewString("_resolveUri"),
+                     kNumArgs,
+                     dart_args);
+}
+
+
 Dart_Handle DartUtils::ResolveUriInWorkingDirectory(Dart_Handle script_uri) {
   const int kNumArgs = 1;
   Dart_Handle dart_args[kNumArgs];