Make Scanner::kNoSourcePos be a true sentinel value

runtime/vm/scanner.h

Index: runtime/vm/scanner.h
diff --git a/runtime/vm/scanner.h b/runtime/vm/scanner.h
index 7558a1d0696d0e66e17c695f70ae621081673536..7a8104081f3408be2f712ef00d0fdac7eeb1f3dc 100644
--- a/runtime/vm/scanner.h
+++ b/runtime/vm/scanner.h
@@ -45,7 +45,11 @@ class Scanner : ValueObject {
   };
 
   // Dummy token index reflecting an unknown source position.
-  static const intptr_t kNoSourcePos = 0;
+  static const intptr_t kNoSourcePos = -1;
+
+  static bool ValidSourcePosition(intptr_t token_pos) {

[hausner, 2016/01/05 22:30:50]

I'd expect ValidSourcePosition() to check whether it is indeed a value that
represents a position, which does not include kNoSourcePos. Maybe this function
is not needed. We only use it in ASSERTS it seems. Just get rid of those
asserts? 

[Cutch, 2016/01/06 15:42:19]

Maybe I should rename the function to be clearer. I considered removing the
ASSERTs too but then I thought that it might be nice to make sure that we never
set a token position that isn't >= 0 or equal to the sentinel value.

+    return (token_pos >= 0) || (token_pos == kNoSourcePos);
+  }
 
   typedef ZoneGrowableArray<TokenDescriptor> GrowableTokenStream;