perf(dart/transform): Only process deferred libs when necessary

Previously, every .dart file in a package was processed to ensure proper
initialization of deferred loaded libraries.

Update the transformer to avoid processing libraries which we know do
not import any deferred libraries.

Closes #6745

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

@@ -6,6 +6,7 @@ const SETUP_METHOD_NAME = 'initReflector';
 const REFLECTOR_VAR_NAME = 'reflector';
 const TRANSFORM_DYNAMIC_MODE = 'transform_dynamic';
 const CSS_EXTENSION = '.css';
+const DEFERRED_EXTENSION = '.dart.deferredCount';
 const SHIMMED_STYLESHEET_EXTENSION = '.css.shim.dart';
 const NON_SHIMMED_STYLESHEET_EXTENSION = '.css.dart';
 const META_EXTENSION = '.ng_meta.json';
@@ -21,8 +22,10 @@ const TEMPLATE_EXTENSION = '.template.dart';
 
 /// Note that due to the implementation of `_toExtension`, ordering is
 /// important. For example, putting '.dart' first in this list will cause
-/// incorrect behavior.
+/// incorrect behavior because it will (incompletely) match '.template.dart'
+/// files.
 const ALL_EXTENSIONS = const [
+  DEFERRED_EXTENSION,
   META_EXTENSION,
   SUMMARY_META_EXTENSION,
   TEMPLATE_EXTENSION,
@@ -36,6 +39,7 @@ const ALL_EXTENSIONS = const [
 /// any files named like transformer outputs will be reported as generated.
 bool isGenerated(String uri) {
   return const [
+    DEFERRED_EXTENSION,
     META_EXTENSION,
     NON_SHIMMED_STYLESHEET_EXTENSION,
     SHIMMED_STYLESHEET_EXTENSION,
@@ -44,6 +48,10 @@ bool isGenerated(String uri) {
   ].any((ext) => uri.endsWith(ext));
 }
 
+/// Returns `uri` with its extension updated to [DEFERRED_EXTENSION].
+String toDeferredExtension(String uri) =>
+    _toExtension(uri, ALL_EXTENSIONS, DEFERRED_EXTENSION);
+
 /// 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

@@ -14,7 +14,7 @@ import 'rewriter.dart';
 /// Transformer responsible for rewriting deferred library loads to enable
 /// initializing the reflector in a deferred way to keep the code with the
 /// deferred library.
-class DeferredRewriter extends Transformer implements LazyTransformer {
+class DeferredRewriter extends AggregateTransformer implements LazyTransformer {
   final TransformerOptions options;
 
   DeferredRewriter(this.options);
@@ -25,23 +25,60 @@ class DeferredRewriter extends Transformer implements LazyTransformer {
   }
 
   @override
-  bool isPrimary(AssetId id) =>
-      id.extension.endsWith('dart') && _isNotGenerated(id);
+  dynamic classifyPrimary(AssetId id) {
+    // Map <name>.dart and <name>.dart.deferredCount => <name>.
+    // Anything else to `null`.
+    var extension = null;
+    if (id.path.endsWith(DEFERRED_EXTENSION)) {
+      extension = DEFERRED_EXTENSION;
+    } else if (id.path.endsWith('.dart') && !isGenerated(id.path)) {
+      extension = '.dart';
+    } else {
+      return null;
+    }
+    return id.path.substring(0, id.path.length - extension.length);
+  }
 
   @override
-  Future apply(Transform transform) async {
+  Future apply(AggregateTransform transform) async {
     return zone.exec(() async {
-      var asset = transform.primaryInput;
-      var reader = new AssetReader.fromTransform(transform);
-      var transformedCode = await rewriteDeferredLibraries(reader, asset.id);
+      final dartAsset = await _assetToProcess(transform);
+      if (dartAsset == null) return;
+
+      var transformedCode = await rewriteDeferredLibraries(
+          new AssetReader.fromTransform(transform), dartAsset.id);
       if (transformedCode != null) {
-        transform.addOutput(new Asset.fromString(asset.id, transformedCode));
+        transform
+            .addOutput(new Asset.fromString(dartAsset.id, transformedCode));
       }
     }, log: transform.logger);
   }
-}
 
-bool _isNotGenerated(AssetId id) => !isGenerated(id.path);
+  /// Returns the asset we need to process or `null` if none exists.
+  ///
+  /// Consumes the .dart.deferredCount asset if it is present.
+  Future<Asset> _assetToProcess(AggregateTransform transform) async {
+    // We only need to process .dart files that have an associated
+    // .dart.deferredCount file with a value != "0".
+    //
+    // The .dart.deferredCount file is generated by a previous phase for files
+    // which have deferred imports. An absent .dart.deferredCount asset is the
+    // treated the same as a .dart.deferredCount asset with value "0".
+    var dartAsset, deferredCountAsset;
+    await for (Asset a in transform.primaryInputs) {
+      if (a.id.path.endsWith(DEFERRED_EXTENSION)) {
+        deferredCountAsset = a;
+      } else if (a.id.path.endsWith('.dart')) {
+        dartAsset = a;
+      }
+    }
+    if (deferredCountAsset == null) return null;
+    // No longer necessary.
+    transform.consumePrimary(deferredCountAsset.id);
+    if ((await deferredCountAsset.readAsString()) == "0") return null;
+    return dartAsset;
+  }
+}
 
 // Visible for testing
 Future<String> rewriteDeferredLibraries(AssetReader reader, AssetId id) async {

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

@@ -33,6 +33,7 @@ class DirectiveProcessor extends Transformer implements LazyTransformer {
 
   @override
   declareOutputs(DeclaringTransform transform) {
+    transform.declareOutput(_deferredAssetId(transform.primaryId));
     transform.declareOutput(_ngSummaryAssetId(transform.primaryId));
   }
 
@@ -49,6 +50,17 @@ class DirectiveProcessor extends Transformer implements LazyTransformer {
       }
       transform.addOutput(new Asset.fromString(
           _ngSummaryAssetId(primaryId), _encoder.convert(ngMeta.toJson())));
+
+      var deferredCount = 0;
+      if (ngMeta.ngDeps != null) {
+        deferredCount = ngMeta.ngDeps.imports.where((i) => i.isDeferred).length;
+      }
+      if (deferredCount > 0) {
+        // The existence of this file with the value != "0" signals
+        // DeferredRewriter that the associated .dart file needs attention.
+        transform.addOutput(new Asset.fromString(
+            _deferredAssetId(primaryId), deferredCount.toString()));
+      }
     }, log: transform.logger);
   }
 }
@@ -57,3 +69,8 @@ AssetId _ngSummaryAssetId(AssetId primaryInputId) {
   return new AssetId(
       primaryInputId.package, toSummaryExtension(primaryInputId.path));
 }
+
+AssetId _deferredAssetId(AssetId primaryInputId) {
+  return new AssetId(
+      primaryInputId.package, toDeferredExtension(primaryInputId.path));
+}