perf(dart/transform): Avoid unnecessary reads for files with no view

In the `TemplateCompiler` phase, avoid reading in the `.ng_meta.json` files of
imported libraries when we can determine that the file we are processing
does not define any `View`s.

Closes angular#6183

Author: Tim Blasi

modules_dart/transform/lib/src/transform/template_compiler/compile_data_creator.dart

@@ -19,6 +19,12 @@ import 'package:barback/barback.dart';
 ///
 /// The returned value wraps the [NgDepsModel] at `assetId` as well as these
 /// created objects.
+///
+/// `platformDirectives` is an optional [List] containing names of [Directive]s
+/// which should be available to all [View]s in this app.
+///
+/// `platformPipes` is an optional [List] containing names of [Pipe]s which
+/// should be available to all [View]s in this app.
 Future<CompileDataResults> createCompileData(
     AssetReader reader,
     AssetId assetId,
@@ -65,9 +71,17 @@ class _CompileDataCreator {
   NgDepsModel get ngDeps => ngMeta.ngDeps;
 
   Future<CompileDataResults> createCompileData() async {
-    if (ngDeps == null || ngDeps.reflectables == null) {
-      return new CompileDataResults._(ngMeta, const {});
-    }
+    var hasTemplate = ngDeps != null &&
+        ngDeps.reflectables != null &&
+        ngDeps.reflectables.any((reflectable) {
+          if (ngMeta.types.containsKey(reflectable.name)) {
+            final metadata = ngMeta.types[reflectable.name];
+            return metadata is CompileDirectiveMetadata &&
+                metadata.template != null;
+          }
+          return false;
+        });
+    if (!hasTemplate) return new CompileDataResults._(ngMeta, const {});
 
     final compileData =
         <ReflectionInfoModel, NormalizedComponentWithViewDirectives>{};

modules_dart/transform/lib/src/transform/template_compiler/generator.dart

@@ -22,9 +22,17 @@ import 'reflection/processor.dart' as reg;
 import 'reflection/reflection_capabilities.dart';
 import 'compile_data_creator.dart';
 
-/// Reads the `.ng_deps.dart` file represented by `assetId` and parses any
-/// Angular 2 `View` annotations it declares to generate `getter`s,
-/// `setter`s, and `method`s that would otherwise be reflectively accessed.
+/// Generates `.ng_deps.dart` files to initialize the Angular2 system.
+///
+/// Processes the `.ng_summary.json` file represented by `assetId`
+/// `createCompileData`.
+/// Uses the resulting `NgMeta` object to generate
+/// `getter`s, `setter`s, and `method`s that would otherwise need to be
+/// reflectively accessed.
+/// Passes the resulting `NormalizedComponentWithViewDirectives` instances
+/// to the `TemplateCompiler` to generate compiled template(s).
+/// Uses the resulting `NgDeps` object to generate a .ng_deps.dart file which
+/// initializes the Angular2 reflective system.
 ///
 /// This method assumes a {@link DomAdapter} has been registered.
 Future<Outputs> processTemplates(AssetReader reader, AssetId assetId,