Implement a 'trial parse' step, that will abort pre-parsing excessively

src/scanner-character-streams.h

Index: src/scanner-character-streams.h
diff --git a/src/scanner-character-streams.h b/src/scanner-character-streams.h
index 5944d7644572320f33498082ca362d0cdb2df564..79ee0425d9a39318eb67fc68409410e25f39117b 100644
--- a/src/scanner-character-streams.h
+++ b/src/scanner-character-streams.h
@@ -43,12 +43,16 @@ class GenericStringUtf16CharacterStream: public BufferedUtf16CharacterStream {
                                     size_t end_position);
   virtual ~GenericStringUtf16CharacterStream();
 
+  virtual bool SetBookmark();
+  virtual void ResetToBookmark();
+
  protected:
   virtual size_t BufferSeekForward(size_t delta);
   virtual size_t FillBuffer(size_t position);
 
   Handle<String> string_;
   size_t length_;
+  size_t bookmark_;
 };
 
 
@@ -129,6 +133,9 @@ class ExternalTwoByteStringUtf16CharacterStream: public Utf16CharacterStream {
     pos_--;
   }
 
+  virtual bool SetBookmark();
+  virtual void ResetToBookmark();
+
  protected:
   virtual size_t SlowSeekForward(size_t delta) {
     // Fast case always handles seeking.
@@ -140,6 +147,11 @@ class ExternalTwoByteStringUtf16CharacterStream: public Utf16CharacterStream {
   }
   Handle<ExternalTwoByteString> source_;
   const uc16* raw_data_;  // Pointer to the actual array of characters.
+
+ private:
+  static const size_t kNoBookmark = -1;
+
+  size_t bookmark_;
 };
 
 } }  // namespace v8::internal