Check snapshots for feature compatibility.

runtime/vm/snapshot.h

Index: runtime/vm/snapshot.h
diff --git a/runtime/vm/snapshot.h b/runtime/vm/snapshot.h
index 1c0fddb28aa5856cb24c76c509e83e569f50693a..b3c38d999b40734f9fb09abc7f4400f6a6c5b5ed 100644
--- a/runtime/vm/snapshot.h
+++ b/runtime/vm/snapshot.h
@@ -442,6 +442,7 @@ class SnapshotReader : public BaseReader {
 
   // 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
@@ -1020,6 +1021,7 @@ class SnapshotWriter : public BaseWriter {
 
   // 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);