style(dart/transform): Do not format generated code by default

Formatting code requires time and memory during the build -- do not do
it unless explicitly requested via a parameter to the transformer.

Add an entry to the
[wiki](https://github.com/angular/angular/wiki/Angular-2-Dart-Transformer)
describing the added parameter.

modules/angular2/test/transform/integration/all_tests.dart

@@ -14,7 +14,8 @@ main() {
 var formatter = new DartFormatter();
 var transform = new AngularTransformerGroup(new TransformerOptions(
     ['web/index.dart'],
-    reflectionEntryPoints: ['web/index.dart']));
+    reflectionEntryPoints: ['web/index.dart'],
+    formatCode: true));
 
 class IntegrationTestConfig {
   final String name;