[interpreter, debugger] implement bytecode break location iterator.

src/interpreter/source-position-table.h

Index: src/interpreter/source-position-table.h
diff --git a/src/interpreter/source-position-table.h b/src/interpreter/source-position-table.h
index f29f246384d1c1e8146c8124795b7806ea275260..dddf1e16004f1d14030e058dc2ade9c2caf026b7 100644
--- a/src/interpreter/source-position-table.h
+++ b/src/interpreter/source-position-table.h
@@ -35,9 +35,10 @@ class SourcePositionTableBuilder {
     uint32_t source_position_and_type;
   };
 
-  void AssertMonotonic(int bytecode_offset) {
-    DCHECK(entries_.size() == 0 ||
-           entries_.back().bytecode_offset < bytecode_offset);
+  bool AlreadyHasPosition(int bytecode_offset) {

[vogelheim, 2016/02/11 12:33:59]

AlreadyHasPosition is somewhat weaker than AssertMonotonic. Can we no longer
guarantee that the bytecode offsets are monotonically increasing?

[Yang, 2016/02/11 14:16:55]

Correct. We can no longer guarantee that. For example, VisitExpressionStatement
may not emit bytecode if it's an empty statement. In this situation, FCG does
not emit a new position (see PositionsRecorder::WriteRecordedPositions), so we
imitate this behavior.

I'm sure we need to tweak this. The logic for this in FCG is pretty complicated,
and I'm not sure how much of that we need to replicate.

+    // Return whether bytecode offset already has a position assigned.
+    return entries_.size() != 0 &&
+           entries_.back().bytecode_offset == bytecode_offset;
   }
 
   Isolate* isolate_;