Fix incorrect asserts in scanner.

src/scanner.cc

 // Copyright 2010 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 58 matching lines @@
   // contains valid data that comes just after the pushback.
   // We NULL the pushback_limit_ if pushing all the way back to the
   // start of the buffer.
 
   if (pushback_limit_ == NULL) {
     // Enter pushback mode.
     pushback_limit_ = buffer_end_;
     buffer_end_ = buffer_ + kBufferSize;
     buffer_cursor_ = buffer_end_;
   }
-  ASSERT(pushback_limit_ > buffer_);
+  // Ensure that there is room for at least one pushback.
+  ASSERT(buffer_cursor_ > buffer_);
   ASSERT(pos_ > 0);
   buffer_[--buffer_cursor_ - buffer_] = character;
   if (buffer_cursor_ == buffer_) {
     pushback_limit_ = NULL;
   } else if (buffer_cursor_ < pushback_limit_) {
     pushback_limit_ = buffer_cursor_;
   }
   pos_--;
 }
 
 
 bool BufferedUC16CharacterStream::ReadBlock() {
+  buffer_cursor_ = buffer_;
   if (pushback_limit_ != NULL) {
-    buffer_cursor_ = buffer_;
+    // Leave pushback mode.
     buffer_end_ = pushback_limit_;
     pushback_limit_ = NULL;
-    ASSERT(buffer_cursor_ != buffer_end_);
-    return true;
+    // If there were any valid characters left at the
+    // start of the buffer, use those.
+    if (buffer_cursor_ < buffer_end_) return true;
+    // Otherwise read a new block.
   }
   unsigned length = FillBuffer(pos_, kBufferSize);
-  buffer_cursor_ = buffer_;
   buffer_end_ = buffer_ + length;
   return length > 0;
 }
 
 
 unsigned BufferedUC16CharacterStream::SlowSeekForward(unsigned delta) {
   // Leave pushback mode (i.e., ignore that there might be valid data
   // in the buffer before the pushback_limit_ point).
   pushback_limit_ = NULL;
   return BufferSeekForward(delta);
@@ skipping 463 matching lines @@
     text++;
   }
   if (ScannerConstants::kIsIdentifierPart.get(c0_)) return Token::ILLEGAL;
   literal.Complete();
   return token;
 }
 
 
 
 } }  // namespace v8::internal