Support redirecting factory constructors in the VM (issue 3969).

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
===================================================================
--- runtime/vm/raw_object.h	(revision 12790)
+++ runtime/vm/raw_object.h	(working copy)
@@ -25,6 +25,7 @@
   V(PatchClass)                                                                \
   V(Function)                                                                  \
   V(ClosureData)                                                               \
+  V(RedirectionData)                                                           \
   V(Field)                                                                     \
   V(LiteralToken)                                                              \
   V(TokenStream)                                                               \
@@ -634,6 +635,22 @@
 };
 
 
+class RawRedirectionData : public RawObject {
+ private:
+  RAW_HEAP_OBJECT_IMPLEMENTATION(RedirectionData);
+
+  RawObject** from() {
+    return reinterpret_cast<RawObject**>(&ptr()->type_);
+  }
+  RawType* type_;
+  RawString* identifier_;
+  RawFunction* target_;
+  RawObject** to() {
+    return reinterpret_cast<RawObject**>(&ptr()->target_);
+  }
+};
+
+
 class RawField : public RawObject {
   RAW_HEAP_OBJECT_IMPLEMENTATION(Field);