Reapply "Use a single file for app snapshots."

runtime/vm/snapshot.cc

Index: runtime/vm/snapshot.cc
diff --git a/runtime/vm/snapshot.cc b/runtime/vm/snapshot.cc
index 0062dacd54416bf28618fe500a12d7036128c274..08cf77ce3a9a7299cf79ad77175343a800e8f584 100644
--- a/runtime/vm/snapshot.cc
+++ b/runtime/vm/snapshot.cc
@@ -704,7 +704,10 @@ static void EnsureIdentifier(char* label) {
 }
 
 
-void AssemblyInstructionsWriter::Write() {
+void AssemblyInstructionsWriter::Write(uint8_t* vmisolate_buffer,
+                                       intptr_t vmisolate_length,
+                                       uint8_t* isolate_buffer,
+                                       intptr_t isolate_length) {
   Thread* thread = Thread::Current();
   Zone* zone = thread->zone();
   NOT_IN_PRODUCT(TimelineDurationScope tds(thread,
@@ -849,10 +852,28 @@ void AssemblyInstructionsWriter::Write() {
       WriteWordLiteralData(*cursor);
     }
   }
+
+
+  assembly_stream_.Print(".globl _kVmIsolateSnapshot\n");
+  assembly_stream_.Print(".balign %" Pd ", 0\n", VirtualMemory::PageSize());
+  assembly_stream_.Print("_kVmIsolateSnapshot:\n");
+  for (intptr_t i = 0; i < vmisolate_length; i++) {
+    assembly_stream_.Print(".byte %" Pd "\n", vmisolate_buffer[i]);
+  }
+
+  assembly_stream_.Print(".globl _kIsolateSnapshot\n");
+  assembly_stream_.Print(".balign %" Pd ", 0\n", VirtualMemory::PageSize());
+  assembly_stream_.Print("_kIsolateSnapshot:\n");
+  for (intptr_t i = 0; i < isolate_length; i++) {
+    assembly_stream_.Print(".byte %" Pd "\n", isolate_buffer[i]);
+  }
 }
 
 
-void BlobInstructionsWriter::Write() {
+void BlobInstructionsWriter::Write(uint8_t* vmisolate_buffer,
+                                   intptr_t vmisolate_len,
+                                   uint8_t* isolate_buffer,
+                                   intptr_t isolate_length) {
   Thread* thread = Thread::Current();
   Zone* zone = thread->zone();
   NOT_IN_PRODUCT(TimelineDurationScope tds(thread,