Implement rewind: drop one or more frames from the debugger.

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index 9097b1d65012b029470e6b3667bfb7efeb2338ad..8ca70b7686a3488d765abec508e8f2910e1de2f0 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -1197,7 +1197,8 @@ class RawPcDescriptors : public RawObject {
     kUnoptStaticCall = kIcCall << 1,       // Call to a known target via stub.
     kRuntimeCall = kUnoptStaticCall << 1,  // Runtime call.
     kOsrEntry = kRuntimeCall << 1,         // OSR entry point in unopt. code.
-    kOther = kOsrEntry << 1,
+    kRewind = kOsrEntry << 1,              // Call rewind target address.
+    kOther = kRewind << 1,
     kLastKind = kOther,
     kAnyKind = -1
   };