Replace dart_core.js with actual compiled SDK

lib/src/options.dart

Index: lib/src/options.dart
diff --git a/lib/src/options.dart b/lib/src/options.dart
index 8f42559b7ca71fda09d7600e9d42eef59ae8edfa..cb7d88beb1591ab299b7fb71af99dacbdcdab1f1 100644
--- a/lib/src/options.dart
+++ b/lib/src/options.dart
@@ -196,7 +196,7 @@ class CompilerOptions implements RulesOptions, ResolverOptions, JSCodeOptions {
   @override
   final bool emitSourceMaps;
 
-  /// Location for runtime files, such as `dart_core.js`. By default this is
+  /// Location for runtime files, such as `dart_runtime.js`. By default this is
   /// inferred to be under `lib/runtime/` in the location of the `dev_compiler`
   /// package (if we can infer where that is located).
   final String runtimeDir;