feat(dart/transform): Do not re-process generated files

In the `DeferredRewriter` phase, do not process files which were
generated by the transformer, since these will never need to be
rewritten.

modules_dart/transform/lib/src/transform/common/names.dart

@@ -30,6 +30,20 @@ const ALL_EXTENSIONS = const [
   '.dart'
 ];
 
+/// Whether `uri` was created by a transform phase.
+///
+/// This may return false positives for problematic inputs.
+/// This just tests file extensions known to be created by the transformer, so
+/// any files named like transformer outputs will be reported as generated.
+bool isGenerated(String uri) {
+  return const [
+    DEPS_EXTENSION,
+    NON_SHIMMED_STYLESHEET_EXTENSION,
+    SHIMMED_STYLESHEET_EXTENSION,
+    TEMPLATE_EXTENSION,
+  ].any((ext) => uri.endsWith(ext));
+}
+
 /// Returns `uri` with its extension updated to [META_EXTENSION].
 String toMetaExtension(String uri) =>
     _toExtension(uri, ALL_EXTENSIONS, META_EXTENSION);

modules_dart/transform/lib/src/transform/deferred_rewriter/transformer.dart

@@ -21,7 +21,7 @@ class DeferredRewriter extends Transformer {
 
   @override
   bool isPrimary(AssetId id) =>
-      id.extension.endsWith('dart') && !id.path.endsWith(DEPS_EXTENSION);
+      id.extension.endsWith('dart') && _isNotGenerated(id);
 
   @override
   Future apply(Transform transform) async {
@@ -36,6 +36,8 @@ class DeferredRewriter extends Transformer {
   }
 }
 
+bool _isNotGenerated(AssetId id) => !isGenerated(id.path);
+
 // Visible for testing
 Future<String> rewriteDeferredLibraries(AssetReader reader, AssetId id) async {
   var rewriter = new Rewriter(id, reader);