Make Scanner::kNoSourcePos be a true sentinel value

runtime/vm/flow_graph_compiler_x64.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"  // Needed here to get TARGET_ARCH_X64.
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/flow_graph_compiler.h"
 
 #include "vm/ast_printer.h"
@@ skipping 659 matching lines @@
 // - RDX: instantiator type arguments or raw_null.
 // Returns:
 // - object in RAX for successful assignable check (or throws TypeError).
 // Performance notes: positive checks must be quick, negative checks can be slow
 // as they throw an exception.
 void FlowGraphCompiler::GenerateAssertAssignable(intptr_t token_pos,
                                                  intptr_t deopt_id,
                                                  const AbstractType& dst_type,
                                                  const String& dst_name,
                                                  LocationSummary* locs) {
-  ASSERT(token_pos >= 0);
+  ASSERT(Scanner::ValidSourcePosition(token_pos));
   ASSERT(!dst_type.IsNull());
   ASSERT(dst_type.IsFinalized());
   // Assignable check is skipped in FlowGraphBuilder, not here.
   ASSERT(dst_type.IsMalformedOrMalbounded() ||
          (!dst_type.IsDynamicType() && !dst_type.IsObjectType()));
   __ pushq(RDX);  // Store instantiator type arguments.
   // A null object is always assignable and is returned as result.
   Label is_assignable, runtime_call;
   __ CompareObject(RAX, Object::null_object());
   __ j(EQUAL, &is_assignable);
@@ skipping 1120 matching lines @@
   __ movups(reg, Address(RSP, 0));
   __ AddImmediate(RSP, Immediate(kFpuRegisterSize));
 }
 
 
 #undef __
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64