Add support for importing using "package:" to the standalone VM.

runtime/bin/builtin.dart

Index: runtime/bin/builtin.dart
diff --git a/runtime/bin/builtin.dart b/runtime/bin/builtin.dart
index 2841e57b7af2a56d1a012a34246e5bf15f4fffb3..b068a305a279fa7b7349f2137e2cfcae501dfe14 100644
--- a/runtime/bin/builtin.dart
+++ b/runtime/bin/builtin.dart
@@ -27,6 +27,10 @@ class _Logger {
 // http://blogs.msdn.com/b/ie/archive/2006/12/06/file-uris-in-windows.aspx
 var _is_windows;
 
+// The URI that the entrypoint script was loaded from. Remembered so that
+// package imports can be resolved relative to it.
+Uri _entrypoint;
+
 void _logResolution(String msg) {
   final enabled = false;
   if (enabled) {
@@ -54,9 +58,10 @@ String _resolveScriptUri(String cwd, String scriptName, bool windows) {
     _logResolution("## scriptName: $scriptName");
   }
   var base = new Uri(scheme: "file", path: cwd.endsWith("/") ? cwd : "$cwd/");
-  var resolved = base.resolve(scriptName);
-  _logResolution("# Resolved to: $resolved");
-  return resolved.toString();
+  _entrypoint = base.resolve(scriptName);
+  _logResolution("# Resolved script to: $_entrypoint");
+
+  return _entrypoint.toString();
 }
 
 String _resolveUri(String base, String userString) {
@@ -84,11 +89,19 @@ String _resolveExtensionUri(String base, String userString) {
 String _filePathFromUri(String userUri) {
   var uri = new Uri.fromString(userUri);
   _logResolution("# Getting file path from: $uri");
-  if ("file" != uri.scheme) {
-    // Only handling file URIs in standalone binary.
-    _logResolution("# Not a file URI.");
-    throw "Not a file uri: $uri";
+
+  switch (uri.scheme) {
+  case 'file':    return _filePathFromFileUri(uri);
+  case 'package': return _filePathFromPackageUri(uri);
+
+  default:
+    // Only handling file and package URIs in standalone binary.
+    _logResolution("# Not a file or package URI.");
+    throw "Not a known scheme: $uri";
   }
+}
+
+String _filePathFromFileUri(Uri uri) {
   var path = uri.path;
   _logResolution("# Path: $path");
   if (_is_windows) {
@@ -98,3 +111,18 @@ String _filePathFromUri(String userUri) {
   }
   return path;
 }
+
+String _filePathFromPackageUri(Uri uri) {
+  // Get the path to the package directory.
+  var baseDir = _entrypoint.path;
+
+  // Remove any extension.
+  var lastDot = baseDir.lastIndexOf(".");
+  if (lastDot != -1) {
+    baseDir = baseDir.substring(0, lastDot);
+  }
+
+  var path = "$baseDir.packages/${uri.path}";
+  _logResolution("# Package: $path");
+  return path;
+}