feat(dart/transform): Remove unnecessary .ng_deps.dart files

Removes `.ng_deps.dart` files which

1. Do not register any `@Injectable` classes
2. Do not call `initReflector` on any other `.ng_deps.dart` files.

Closes #1929

modules/angular2/src/transform/directive_linker/linker.dart

@@ -11,18 +11,56 @@ import 'package:barback/barback.dart';
 import 'package:code_transformers/assets.dart';
 import 'package:path/path.dart' as path;
 
+/// Checks the `.ng_deps.dart` file represented by `entryPoint` and
+/// determines whether it is necessary to the functioning of the Angular 2
+/// Dart app.
+///
+/// An `.ng_deps.dart` file is not necessary if:
+/// 1. It does not register any `@Injectable` types with the system.
+/// 2. It does not import any libraries whose `.ng_deps.dart` files register
+///    any `@Injectable` types with the system.
+///
+/// Since `@Directive` and `@Component` inherit from `@Injectable`, we know
+/// we will not miss processing any classes annotated with those tags.
+Future<bool> isNecessary(AssetReader reader, AssetId entryPoint) async {
+  var parser = new Parser(reader);
+  NgDeps ngDeps = await parser.parse(entryPoint);
+
+  if (ngDeps.registeredTypes.isNotEmpty) return true;
+
+  // We do not register any @Injectables, do we call any dependencies?
+  var linkedDepsMap =
+      await _processNgImports(reader, entryPoint, _getSortedDeps(ngDeps));
+  return linkedDepsMap.isNotEmpty;
+}
+
+/// Modifies the `.ng_deps.dart` file represented by `entryPoint` to call its
+/// dependencies associated `initReflector` methods.
+///
+/// For example, if entry_point.ng_deps.dart imports dependency.dart, this
+/// will check if dependency.ng_deps.dart exists. If it does, we add:
+///
+/// ```
+/// import 'dependency.ng_deps.dart' as i0;
+/// ...
+/// void setupReflection(reflector) {
+///   ...
+///   i0.initReflector(reflector);
+/// }
+/// ```
 Future<String> linkNgDeps(AssetReader reader, AssetId entryPoint) async {
   var parser = new Parser(reader);
   NgDeps ngDeps = await parser.parse(entryPoint);
+
   if (ngDeps == null) return null;
 
-  var allDeps = <UriBasedDirective>[]
-    ..addAll(ngDeps.imports)
-    ..addAll(ngDeps.exports)
-    ..sort((a, b) => a.end.compareTo(b.end));
+  var allDeps = _getSortedDeps(ngDeps);
   var linkedDepsMap = await _processNgImports(reader, entryPoint, allDeps);
 
-  if (linkedDepsMap.isEmpty) return ngDeps.code;
+  if (linkedDepsMap.isEmpty) {
+    // We are not calling `initReflector` on any other libraries.
+    return ngDeps.code;
+  }
 
   var importBuf = new StringBuffer();
   var declarationBuf = new StringBuffer();
@@ -49,6 +87,15 @@ Future<String> linkNgDeps(AssetReader reader, AssetId entryPoint) async {
       '${code.substring(declarationSeamIdx)}';
 }
 
+/// All `import`s and `export`s in `ngDeps` sorted by order of appearance in
+/// the file.
+List<UriBasedDirective> _getSortedDeps(NgDeps ngDeps) {
+  return <UriBasedDirective>[]
+    ..addAll(ngDeps.imports)
+    ..addAll(ngDeps.exports)
+    ..sort((a, b) => a.end.compareTo(b.end));
+}
+
 String _toDepsUri(String importUri) =>
     '${path.withoutExtension(importUri)}${DEPS_EXTENSION}';
 
@@ -67,10 +114,8 @@ Future<Map<UriBasedDirective, String>> _processNgImports(AssetReader reader,
           .where(_isNotDartDirective)
           .map((UriBasedDirective directive) {
     var ngDepsUri = _toDepsUri(stringLiteralToString(directive.uri));
-    var ngDepsAsset = uriToAssetId(entryPoint, ngDepsUri, logger,
-        null /*
-    span */
-        );
+    var spanArg = null;
+    var ngDepsAsset = uriToAssetId(entryPoint, ngDepsUri, logger, spanArg);
     if (ngDepsAsset == entryPoint) return nullFuture;
     return reader.hasInput(ngDepsAsset).then((hasInput) {
       if (hasInput) {

modules/angular2/src/transform/directive_linker/transformer.dart

@@ -29,8 +29,10 @@ class DirectiveLinker extends Transformer {
       var assetId = transform.primaryInput.id;
       var assetPath = assetId.path;
       var transformedCode = await linkNgDeps(reader, assetId);
-      var formattedCode = formatter.format(transformedCode, uri: assetPath);
-      transform.addOutput(new Asset.fromString(assetId, formattedCode));
+      if (transformedCode != null) {
+        var formattedCode = formatter.format(transformedCode, uri: assetPath);
+        transform.addOutput(new Asset.fromString(assetId, formattedCode));
+      }
     } catch (ex, stackTrace) {
       log.logger.error('Linking ng directives failed.\n'
           'Exception: $ex\n'
@@ -39,3 +41,29 @@ class DirectiveLinker extends Transformer {
     return null;
   }
 }
+
+/// Transformer responsible for removing unnecessary `.ng_deps.dart` files
+/// created by {@link DirectiveProcessor}.
+class EmptyNgDepsRemover extends Transformer {
+  EmptyNgDepsRemover();
+
+  @override
+  bool isPrimary(AssetId id) => id.path.endsWith(DEPS_EXTENSION);
+
+  @override
+  Future apply(Transform transform) async {
+    log.init(transform);
+
+    try {
+      var reader = new AssetReader.fromTransform(transform);
+      if (!(await isNecessary(reader, transform.primaryInput.id))) {
+        transform.consumePrimary();
+      }
+    } catch (ex, stackTrace) {
+      log.logger.error('Removing unnecessary ng deps failed.\n'
+          'Exception: $ex\n'
+          'Stack Trace: $stackTrace');
+    }
+    return null;
+  }
+}