dart2js: Set inferred return types for async, async*, and sync* methods.

pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart

Index: pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
diff --git a/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart b/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
index 576e632b5725654d345d232e4498e9beac88a9e2..4a03bafed62e2f1b15f56ab4ae039827043fb166 100644
--- a/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
@@ -233,6 +233,27 @@ class TypeInformationSystem extends TypeSystem<TypeInformation> {
         getConcreteTypeFor(compiler.typesTask.dynamicType);
   }
 
+  TypeInformation asyncFutureTypeCache;
+  TypeInformation get asyncFutureType {
+    if (asyncFutureTypeCache != null) return asyncFutureTypeCache;
+    return asyncFutureTypeCache =
+        getConcreteTypeFor(compiler.typesTask.asyncFutureType);
+  }
+
+  TypeInformation syncStarIterableTypeCache;
+  TypeInformation get syncStarIterableType {
+    if (syncStarIterableTypeCache != null) return syncStarIterableTypeCache;
+    return syncStarIterableTypeCache =
+        getConcreteTypeFor(compiler.typesTask.syncStarIterableType);
+  }
+
+  TypeInformation asyncStarStreamTypeCache;
+  TypeInformation get asyncStarStreamType {
+    if (asyncStarStreamTypeCache != null) return asyncStarStreamTypeCache;
+    return asyncStarStreamTypeCache =
+        getConcreteTypeFor(compiler.typesTask.asyncStarStreamType);
+  }
+
   TypeInformation nonNullEmptyType;
 
   TypeInformation stringLiteralType(ast.DartString value) {