Don't test directly against Scanner::kNoSourcePos instead use >= 0.

runtime/vm/debugger.cc

Index: runtime/vm/debugger.cc
diff --git a/runtime/vm/debugger.cc b/runtime/vm/debugger.cc
index 39dc8a8e863e2a2a3f0f892e2226eefc068a6e47..c533e3b44bd4836096979077e0b1e53fbe7b54a9 100644
--- a/runtime/vm/debugger.cc
+++ b/runtime/vm/debugger.cc
@@ -1740,7 +1740,7 @@ intptr_t Debugger::ResolveBreakpointPos(const Function& func,
   PcDescriptors::Iterator iter(desc, kSafepointKind);
   while (iter.MoveNext()) {
     const intptr_t pos = iter.TokenPos();
-    if ((pos == Scanner::kNoSourcePos) ||
+    if ((pos < 0) ||
         (pos < requested_token_pos) ||
         (pos > last_token_pos)) {
       // Token is not in the target range.
@@ -1786,7 +1786,7 @@ intptr_t Debugger::ResolveBreakpointPos(const Function& func,
     PcDescriptors::Iterator iter(desc, kSafepointKind);
     while (iter.MoveNext()) {
       const intptr_t pos = iter.TokenPos();
-      if ((pos == Scanner::kNoSourcePos) ||
+      if ((pos < 0) ||
           (pos < begin_pos) ||
           (pos > end_of_line_pos)) {
         // Token is not on same line as best fit.
@@ -1827,7 +1827,7 @@ intptr_t Debugger::ResolveBreakpointPos(const Function& func,
 
 void Debugger::MakeCodeBreakpointAt(const Function& func,
                                     BreakpointLocation* loc) {
-  ASSERT(loc->token_pos_ != Scanner::kNoSourcePos);
+  ASSERT(loc->token_pos_ >= 0);
   ASSERT((loc != NULL) && loc->IsResolved());
   ASSERT(!func.HasOptimizedCode());
   Code& code = Code::Handle(func.unoptimized_code());
@@ -2641,7 +2641,7 @@ RawError* Debugger::DebuggerStepCallback() {
   if (!frame->IsDebuggable()) {
     return Error::null();
   }
-  if (frame->TokenPos() == Scanner::kNoSourcePos) {
+  if (frame->TokenPos() < 0) {
     return Error::null();
   }