Remember and reuse derived map for external arrays

src/mirror-debugger.js

Index: src/mirror-debugger.js
diff --git a/src/mirror-debugger.js b/src/mirror-debugger.js
index 416f88794dadf07bbd481ae0f2dc8d152974947d..99e981973e4deb47c269ec2bea78cfea4e1b4cb4 100644
--- a/src/mirror-debugger.js
+++ b/src/mirror-debugger.js
@@ -170,14 +170,15 @@ PropertyKind.Indexed = 2;
 
 // A copy of the PropertyType enum from global.h
 PropertyType = {};
-PropertyType.Normal             = 0;
-PropertyType.Field              = 1;
-PropertyType.ConstantFunction   = 2;
-PropertyType.Callbacks          = 3;
-PropertyType.Interceptor        = 4;
-PropertyType.MapTransition      = 5;
-PropertyType.ConstantTransition = 6;
-PropertyType.NullDescriptor     = 7;
+PropertyType.Normal                  = 0;
+PropertyType.Field                   = 1;
+PropertyType.ConstantFunction        = 2;
+PropertyType.Callbacks               = 3;
+PropertyType.Interceptor             = 4;
+PropertyType.MapTransition           = 5;
+PropertyType.ExternalArrayTransition = 6;
+PropertyType.ConstantTransition      = 7;
+PropertyType.NullDescriptor          = 8;
 
 
 // Different attributes for a property.