Hack serialization of Environment$Extension so that it deserializes, although not correctly. Attempts to use it will still most likely fail, but won't crash.

--
MOS_MIGRATED_REVID=103029603

src/main/java/com/google/devtools/build/lib/syntax/Environment.java

diff --git a/src/main/java/com/google/devtools/build/lib/syntax/Environment.java b/src/main/java/com/google/devtools/build/lib/syntax/Environment.java
index 1bfba3a..89360a7 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/Environment.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/Environment.java
@@ -230,6 +230,11 @@
     BaseExtension(Environment env) {
       this.bindings = ImmutableMap.copyOf(env.globalFrame.bindings);
     }
+
+    // Hack to allow serialization.
+    BaseExtension() {
+      this.bindings = ImmutableMap.of();
+    }
   }
 
   /**
@@ -249,6 +254,12 @@
       this.transitiveContentHashCode = env.getTransitiveContentHashCode();
     }
 
+    // Hack to allow serialization.
+    private Extension() {
+      super();
+      this.transitiveContentHashCode = null;
+    }
+
     @VisibleForTesting // This is only used in one test.
     public String getTransitiveContentHashCode() {
       return transitiveContentHashCode;