Build CodeSourceMap for each code object

runtime/vm/code_descriptors.cc

Index: runtime/vm/code_descriptors.cc
diff --git a/runtime/vm/code_descriptors.cc b/runtime/vm/code_descriptors.cc
index e5fa106746c9ca756c77762b250baf89d9617c61..dad76d97e0179cb0b70c971d282fcc663011ce2a 100644
--- a/runtime/vm/code_descriptors.cc
+++ b/runtime/vm/code_descriptors.cc
@@ -44,6 +44,9 @@ RawPcDescriptors* DescriptorList::FinalizePcDescriptors(uword entry_point) {
 
 void CodeSourceMapBuilder::AddEntry(intptr_t pc_offset,
                                     TokenPosition token_pos) {
+  // Require pc offset to monotonically increase.
+  ASSERT((prev_pc_offset < pc_offset) ||
+         ((prev_pc_offset == 0) && (pc_offset == 0)));
   CodeSourceMap::EncodeInteger(&encoded_data_, pc_offset - prev_pc_offset);
   CodeSourceMap::EncodeInteger(&encoded_data_,
                                token_pos.value() - prev_token_pos);