Fix more errors in DDC SDK.

pkg/dev_compiler/tool/input_sdk/private/isolate_helper.dart

Index: pkg/dev_compiler/tool/input_sdk/private/isolate_helper.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/isolate_helper.dart b/pkg/dev_compiler/tool/input_sdk/private/isolate_helper.dart
index e1e1aae0bc226cacb2db49aeda36d3132714682f..039b14f08a1321e2f099f4d6bf3ec9eedf872fde 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/isolate_helper.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/isolate_helper.dart
@@ -746,6 +746,14 @@ class IsolateNatives {
 
   static String thisScript = computeThisScript();
 
+  /// Returns the base path added to Uri.base to resolve `package:` Uris.
+  ///
+  /// This is used by `Isolate.resolvePackageUri` to load resources. The default
+  /// value is `packages/` but users can override this by using the
+  /// `defaultPackagesBase` hook.
+  static String get packagesBase =>
+      JS('String', r'#.defaultPackagesBase || "packages/"', global);
+
   /// Associates an ID with a native worker object.
   static final Expando<int> workerIds = new Expando<int>();