Update from Google.

--
MOE_MIGRATED_REVID=85702957

src/main/java/com/google/devtools/build/skyframe/EvaluableGraph.java

diff --git a/src/main/java/com/google/devtools/build/skyframe/EvaluableGraph.java b/src/main/java/com/google/devtools/build/skyframe/EvaluableGraph.java
new file mode 100644
index 0000000..3d9a934
--- /dev/null
+++ b/src/main/java/com/google/devtools/build/skyframe/EvaluableGraph.java
@@ -0,0 +1,26 @@
+// Copyright 2014 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package com.google.devtools.build.skyframe;
+
+/**
+ * Interface between a single version of the graph and the evaluator. Supports mutation of that
+ * single version of the graph.
+ */
+interface EvaluableGraph extends QueryableGraph {
+  /**
+   * Creates a new node with the specified key if it does not exist yet. Returns the node entry
+   * (either the existing one or the one just created), never {@code null}.
+   */
+  NodeEntry createIfAbsent(SkyKey key);
+}