dart2js: Use JavaScript asts instead of CodeBuffers when assembling output units.

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 12f204e79343dcd347822df75d6c615fbcd92a04..8603b208c012be7a17ba3ac467a1047170ab8bfa 100644
--- a/pkg/js_ast/lib/src/builder.dart
+++ b/pkg/js_ast/lib/src/builder.dart
@@ -329,6 +329,8 @@ class JsBuilder {
   /// [escapedString].
   LiteralString string(String value) => new LiteralString('"$value"');
 
+  LiteralString name(String name) => new LiteralString(name);
+
   LiteralNumber number(num value) => new LiteralNumber('$value');
 
   LiteralBool boolean(bool value) => new LiteralBool(value);