Use a single file for app snapshots.

runtime/vm/snapshot.h

Index: runtime/vm/snapshot.h
diff --git a/runtime/vm/snapshot.h b/runtime/vm/snapshot.h
index 3f767b8eb25161de71f96942513aa897f156146f..020f4485a96efe75537e2ae74bc8ec176c40117a 100644
--- a/runtime/vm/snapshot.h
+++ b/runtime/vm/snapshot.h
@@ -750,7 +750,10 @@ class InstructionsWriter : public ZoneAllocated {
 
   int32_t GetObjectOffsetFor(RawObject* raw_object);
 
-  virtual void Write() = 0;
+  virtual void Write(uint8_t* vmisolate_buffer,
+                     intptr_t vmisolate_length,
+                     uint8_t* isolate_buffer,
+                     intptr_t isolate_length) = 0;
   virtual intptr_t text_size() = 0;
   virtual intptr_t data_size() = 0;
 
@@ -803,7 +806,10 @@ class AssemblyInstructionsWriter : public InstructionsWriter {
       data_size_(0) {
   }
 
-  virtual void Write();
+  virtual void Write(uint8_t* vmisolate_buffer,
+                     intptr_t vmisolate_length,
+                     uint8_t* isolate_buffer,
+                     intptr_t isolate_length);
   virtual intptr_t text_size() { return text_size_; }
   virtual intptr_t data_size() { return data_size_; }
 
@@ -849,7 +855,10 @@ class BlobInstructionsWriter : public InstructionsWriter {
       rodata_blob_stream_(rodata_blob_buffer, alloc, initial_size) {
   }
 
-  virtual void Write();
+  virtual void Write(uint8_t* vmisolate_buffer,
+                     intptr_t vmisolate_length,
+                     uint8_t* isolate_buffer,
+                     intptr_t isolate_length);
   virtual intptr_t text_size() { return InstructionsBlobSize(); }
   virtual intptr_t data_size() { return RodataBlobSize(); }