Introduce CodeSourceMap object to hold pc -> token position mappings

runtime/vm/snapshot.cc

Index: runtime/vm/snapshot.cc
diff --git a/runtime/vm/snapshot.cc b/runtime/vm/snapshot.cc
index 80c034b01bcc7df2a1808a67bfeaf43679ffa5fd..a1c1d5c259906da39be94c32abf7f1022efa013b 100644
--- a/runtime/vm/snapshot.cc
+++ b/runtime/vm/snapshot.cc
@@ -786,6 +786,17 @@ RawPcDescriptors* SnapshotReader::NewPcDescriptors(intptr_t len) {
 }
 
 
+RawCodeSourceMap* SnapshotReader::NewCodeSourceMap(intptr_t len) {
+  ASSERT(kind_ == Snapshot::kFull);
+  ASSERT_NO_SAFEPOINT_SCOPE();
+  RawCodeSourceMap* obj = reinterpret_cast<RawCodeSourceMap*>(
+      AllocateUninitialized(kCodeSourceMapCid,
+                            CodeSourceMap::InstanceSize(len)));
+  obj->ptr()->length_ = len;
+  return obj;
+}
+
+
 RawStackmap* SnapshotReader::NewStackmap(intptr_t len) {
   ASSERT(kind_ == Snapshot::kFull);
   ASSERT_NO_SAFEPOINT_SCOPE();