Add VM class for Map/LinkedHashMap.

runtime/vm/snapshot.h

Index: runtime/vm/snapshot.h
===================================================================
--- runtime/vm/snapshot.h	(revision 38122)
+++ runtime/vm/snapshot.h	(working copy)
@@ -42,6 +42,7 @@
 class RawRedirectionData;
 class RawFunction;
 class RawGrowableObjectArray;
+class RawLinkedHashMap;
 class RawFloat32x4;
 class RawFloat64x2;
 class RawInt32x4;
@@ -373,6 +374,7 @@
   friend class RedirectionData;
   friend class Function;
   friend class GrowableObjectArray;
+  friend class LinkedHashMap;
   friend class ImmutableArray;
   friend class JSRegExp;
   friend class LanguageError;
@@ -572,6 +574,7 @@
   friend class RawClass;
   friend class RawClosureData;
   friend class RawGrowableObjectArray;
+  friend class RawLinkedHashMap;
   friend class RawImmutableArray;
   friend class RawJSRegExp;
   friend class RawLibrary;