Check snapshots for feature compatibility.

runtime/vm/snapshot.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_SNAPSHOT_H_
 #define VM_SNAPSHOT_H_
 
 #include "platform/assert.h"
 #include "vm/allocation.h"
 #include "vm/bitfield.h"
@@ skipping 424 matching lines @@
   Object* GetBackRef(intptr_t id);
 
   // Read a full snap shot.
   RawApiError* ReadFullSnapshot();
 
   // Read a script snap shot.
   RawObject* ReadScriptSnapshot();
 
   // Read version number of snapshot and verify.
   RawApiError* VerifyVersion();
+  RawApiError* VerifyFeatures();
 
   // Helper functions for creating uninitialized versions
   // of various object types. These are used when reading a
   // full snapshot.
   RawArray* NewArray(intptr_t len);
   RawImmutableArray* NewImmutableArray(intptr_t len);
   RawOneByteString* NewOneByteString(intptr_t len);
   RawTwoByteString* NewTwoByteString(intptr_t len);
   RawTypeArguments* NewTypeArguments(intptr_t len);
   RawTokenStream* NewTokenStream(intptr_t len);
@@ skipping 558 matching lines @@
   const char* exception_msg() const { return exception_msg_; }
   void set_exception_msg(const char* msg) {
     exception_msg_ = msg;
   }
   bool can_send_any_object() const { return can_send_any_object_; }
   bool writing_vm_isolate() const { return writing_vm_isolate_; }
   void ThrowException(Exceptions::ExceptionType type, const char* msg);
 
   // Write a version string for the snapshot.
   void WriteVersion();
+  void WriteFeatures();

[siva, 2016/05/16 16:48:00]

The comment is incorrect as it writes more than the version information. Why did
you choose to make it into two separate functions instead of going with one;

void WriteHeaderInfo();
void VerifyHeaderInfo();

[rmacnak, 2016/05/16 20:59:56]

I started out thinking the features string wouldn't be used in all snapshot
kinds, before I realized the mode factors in even without code. Merged.

 
   int32_t GetInstructionsId(RawInstructions* instructions, RawCode* code) {
     return instructions_writer_->GetOffsetFor(instructions, code);
   }
 
   int32_t GetObjectId(RawObject* raw) {
     return instructions_writer_->GetObjectOffsetFor(raw);
   }
 
   void WriteFunctionId(RawFunction* func, bool owner_is_class);
@@ skipping 182 matching lines @@
  private:
   SnapshotWriter* writer_;
   bool as_references_;
 
   DISALLOW_COPY_AND_ASSIGN(SnapshotWriterVisitor);
 };
 
 }  // namespace dart
 
 #endif  // VM_SNAPSHOT_H_