fix(transformer): Serializing execution of transformers

When executing under pub serve the order of apply for the transformers may not be the same as the phase ordering. This was causing deadlocks for the transformers sharing a resolver as the read of an asset for one transformer was invoking the apply for a different transformer, which would then deadlock waiting for the resolver.

Closes #889

Author: Pete Blois

lib/transformer.dart

@@ -1,5 +1,6 @@
 library angular.transformer;
 
+import 'dart:async';
 import 'dart:io';
 import 'package:angular/tools/transformer/expression_generator.dart';
 import 'package:angular/tools/transformer/metadata_generator.dart';
@@ -123,9 +124,37 @@ List<List<Transformer>> _createPhases(TransformOptions options) {
   var resolvers = new Resolvers(options.sdkDirectory);
   return [
     [new HtmlDartReferencesGenerator(options)],
-    [new ExpressionGenerator(options, resolvers)],
-    [new di.InjectorGenerator(options.diOptions, resolvers)],
-    [new MetadataGenerator(options, resolvers)],
-    [new StaticAngularGenerator(options, resolvers)],
+    [new _SerialTransformer([
+      new ExpressionGenerator(options, resolvers),
+      new di.InjectorGenerator(options.diOptions, resolvers),
+      new MetadataGenerator(options, resolvers),
+      new StaticAngularGenerator(options, resolvers)
+    ])]
   ];
 }
+
+/// Helper which runs a group of transformers serially and ensures that
+/// transformers with shared data are always applied in a specific order.
+///
+/// Transformers which communicate only via assets do not need this additional
+/// synchronization.
+///
+/// This is used by Angular to ensure ordering of references to the cached
+/// resolvers.
+class _SerialTransformer extends Transformer {
+  final Iterable<Transformer> _transformers;
+  _SerialTransformer(this._transformers);
+
+  Future<bool> isPrimary(Asset input) =>
+      Future.wait(_transformers.map((t) => t.isPrimary(input)))
+          .then((l) => l.any((result) => result));
+
+  Future apply(Transform transform) {
+    return Future.forEach(_transformers, (t) {
+      return new Future.value(t.isPrimary(transform.primaryInput))
+        .then((isPrimary) {
+          if (isPrimary) return t.apply(transform);
+        });
+    });
+  }
+}