dart2js: Support natives in the startup emitter.

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 076ff65bb62ee4108eb004c41c6d660088123bce..67ea25c9994272a006bcd60a49e8e7dea2a1a3c7 100644
--- a/pkg/js_ast/lib/src/builder.dart
+++ b/pkg/js_ast/lib/src/builder.dart
@@ -377,6 +377,14 @@ class JsBuilder {
                     List<Expression> arguments) {
     return new Call(new PropertyAccess(receiver, fieldName), arguments);
   }
+
+  ObjectInitializer objectLiteral(Map<String, Expression> map) {
+    List<Property> properties = <Property>[];
+    map.forEach((name, value) {
+      properties.add(new Property(string(name), value));
+    });
+    return new ObjectInitializer(properties);
+  }
 }
 
 LiteralString string(String value) => js.string(value);
@@ -400,6 +408,9 @@ Call propertyCall(Expression receiver,
                   List<Expression> arguments) {
   return js.propertyCall(receiver, fieldName, arguments);
 }
+ObjectInitializer objectLiteral(Map<String, Expression> map) {
+  return js.objectLiteral(map);
+}
 
 class MiniJsParserError {
   MiniJsParserError(this.parser, this.message) { }