feat(dart/transform): Bail early for files with no deferred libraries

In `DeferredRewriter` if we can determine that there are no `deferred`
libraries imported, bail early to avoid parsing the entire file.

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

@@ -14,39 +14,54 @@ import 'package:quiver/iterables.dart' as it;
 class Rewriter {
   final AssetId _entryPoint;
   final AssetReader _reader;
+  final _FindDeferredLibraries _visitor;
 
-  Rewriter(this._entryPoint, this._reader);
+  Rewriter(AssetId entryPoint, AssetReader reader)
+      : _entryPoint = entryPoint,
+        _reader = reader,
+        _visitor = new _FindDeferredLibraries(reader, entryPoint);
 
-  /// Rewrites the provided code by finding all the deferred library imports
-  /// and loadLibrary invocations. Then it removes any libraries that don't
-  /// require angular codegen. For the remaining libraries it rewrites the
-  /// import to the corresponding ng_dep file, and chains a future which
-  /// first initializes the library.
+  /// Rewrites `loadLibrary` calls to initialize libraries once loaded.
+  ///
+  /// 1. Finds all the deferred library imports and loadLibrary invocations in
+  ///    `_entryPoint`
+  /// 2. Removes any libraries that don't require angular codegen.
+  /// 3. For the remaining libraries, rewrites the import to the corresponding
+  ///    `ng_deps.dart` file.
+  /// 4. Chains a future to the `loadLibrary` call which initializes the
+  ///    library.
   ///
   /// To the extent possible, this method does not change line numbers or
   /// offsets in the provided code to facilitate debugging via source maps.
   Future<String> rewrite() async {
     var code = await _reader.readAsString(_entryPoint);
-    var node = parseCompilationUnit(code);
+
+    // If we can determine there are no deferred libraries, avoid parsing the
+    // entire file and bail early.
+    var onlyDirectives = parseDirectives(code, name: _entryPoint.path);
+    if (onlyDirectives == null) return null;
+    onlyDirectives.directives.accept(_visitor);
+    if (_visitor.deferredImports.isEmpty) return null;
+
+    var node = parseCompilationUnit(code, name: _entryPoint.path);
     if (node == null) return null;
 
     return logElapsedAsync(() async {
-      var visitor = new _FindDeferredLibraries(_reader, _entryPoint);
-      node.accept(visitor);
+      node.declarations.accept(_visitor);
       // Look to see if we found any deferred libraries
-      if (!visitor.hasDeferredLibrariesToRewrite()) return null;
+      if (!_visitor.hasDeferredLibrariesToRewrite()) return null;
       // Remove any libraries that don't need angular codegen.
-      await visitor.cull();
+      await _visitor.cull();
       // Check again if there are any deferred libraries.
-      if (!visitor.hasDeferredLibrariesToRewrite()) return null;
+      if (!_visitor.hasDeferredLibrariesToRewrite()) return null;
 
       var compare = (AstNode a, AstNode b) => a.offset - b.offset;
-      visitor.deferredImports.sort(compare);
-      visitor.loadLibraryInvocations.sort(compare);
+      _visitor.deferredImports.sort(compare);
+      _visitor.loadLibraryInvocations.sort(compare);
 
       var buf = new StringBuffer();
       var idx =
-          visitor.deferredImports.fold(0, (int lastIdx, ImportDirective node) {
+          _visitor.deferredImports.fold(0, (int lastIdx, ImportDirective node) {
         buf.write(code.substring(lastIdx, node.offset));
 
         var import = code.substring(node.offset, node.end);
@@ -54,7 +69,7 @@ class Rewriter {
         return node.end;
       });
 
-      idx = visitor.loadLibraryInvocations.fold(idx,
+      idx = _visitor.loadLibraryInvocations.fold(idx,
           (int lastIdx, MethodInvocation node) {
         buf.write(code.substring(lastIdx, node.offset));
         var value = node.realTarget as SimpleIdentifier;

modules_dart/transform/test/transform/deferred_rewriter/all_tests.dart

@@ -44,13 +44,15 @@ void _testRewriteDeferredLibraries(String name, String inputPath) {
           path.dirname(inputPath), 'expected', path.basename(inputPath));
       var expectedId = _assetIdForPath(expectedPath);
 
-      var output = await rewriteDeferredLibraries(reader, inputId);
-      var input = await reader.readAsString(expectedId);
-      if (input == null) {
-        // Null input signals no output. Ensure that is true.
-        expect(output).toBeNull();
+      var actualOutput = await rewriteDeferredLibraries(reader, inputId);
+      var expectedOutput = await reader.readAsString(expectedId);
+      if (expectedOutput == null) {
+        // Null expectedOutput signals no output. Ensure that is true.
+        expect(actualOutput).toBeNull();
       } else {
-        expect(formatter.format(output)).toEqual(formatter.format(input));
+        expect(actualOutput).toBeNotNull();
+        expect(formatter.format(actualOutput))
+            .toEqual(formatter.format(expectedOutput));
       }
     }, log: new RecordingLogger());
   });