dart2js: add compiler builtins to the core-runtime.

pkg/js_ast/lib/src/builder.dart

Index: pkg/js_ast/lib/src/builder.dart
diff --git a/pkg/js_ast/lib/src/builder.dart b/pkg/js_ast/lib/src/builder.dart
index 01532307bca541f7605166de7b8e7b96bf60344b..12f204e79343dcd347822df75d6c615fbcd92a04 100644
--- a/pkg/js_ast/lib/src/builder.dart
+++ b/pkg/js_ast/lib/src/builder.dart
@@ -257,6 +257,15 @@ class JsBuilder {
   }
 
   /**
+   * Creates an Expression template for the given [source].
+   *
+   * The returned template is cached.
+   */
+  Template expressionTemplateFor(String source) {
+    return _findExpressionTemplate(source);
+  }
+
+  /**
    * Creates an Expression template without caching the result.
    */
   Template uncachedExpressionTemplate(String source) {