Make 3-arg form of getattr() work when third arg is None

--
MOS_MIGRATED_REVID=128463789

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

diff --git a/src/main/java/com/google/devtools/build/lib/syntax/MethodLibrary.java b/src/main/java/com/google/devtools/build/lib/syntax/MethodLibrary.java
index 59690f4..f103211 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/MethodLibrary.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/MethodLibrary.java
@@ -2120,7 +2120,7 @@
       parameters = {
         @Param(name = "x", doc = "The struct whose attribute is accessed."),
         @Param(name = "name", doc = "The name of the struct attribute."),
-        @Param(name = "default", defaultValue = "None",
+        @Param(name = "default", defaultValue = "unbound",
             doc = "The default value to return in case the struct "
             + "doesn't have an attribute of the given name.")},
       useLocation = true, useEnvironment = true)
@@ -2133,7 +2133,7 @@
         // 'Real' describes methods with structField() == false. Because DotExpression.eval returned
         // null in this case, we know that structField() cannot return true.
         boolean isRealMethod = hasMethod(obj, name, loc);
-        if (defaultValue != Runtime.NONE && !isRealMethod) {
+        if (defaultValue != Runtime.UNBOUND && !isRealMethod) {
           return defaultValue;
         }
         throw new EvalException(loc, Printer.format("Object of type '%s' has no attribute %r%s",

src/test/java/com/google/devtools/build/lib/syntax/MethodLibraryTest.java

diff --git a/src/test/java/com/google/devtools/build/lib/syntax/MethodLibraryTest.java b/src/test/java/com/google/devtools/build/lib/syntax/MethodLibraryTest.java
index 725a21f..1eaecb7 100644
--- a/src/test/java/com/google/devtools/build/lib/syntax/MethodLibraryTest.java
+++ b/src/test/java/com/google/devtools/build/lib/syntax/MethodLibraryTest.java
@@ -460,7 +460,8 @@
         .testIfExactError(
             "Object of type 'string' has no attribute \"not_there\"",
             "getattr('a string', 'not_there')")
-        .testStatement("getattr('a string', 'not_there', 'use this')", "use this");
+        .testStatement("getattr('a string', 'not_there', 'use this')", "use this")
+        .testStatement("getattr('a string', 'not there', None)", Runtime.NONE);
   }
 
   @Test