Fix clang build failures

runtime/vm/intermediate_language.h

 // 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.
 
 #ifndef VM_INTERMEDIATE_LANGUAGE_H_
 #define VM_INTERMEDIATE_LANGUAGE_H_
 
 #include "vm/allocation.h"
 #include "vm/ast.h"
 #include "vm/growable_array.h"
@@ skipping 51 matching lines @@
   static const char* ToCString(intptr_t token_pos) {
     ASSERT(token_pos < 0);
     switch (token_pos) {
       case Scanner::kNoSourcePos: return "NoSource";
 #define DEFINE_CASE(name, value)                                               \
       case value: return #name;
       CLASSIFYING_TOKEN_POSITIONS(DEFINE_CASE);
 #undef DEFINE_CASE
       default:
         UNIMPLEMENTED();
+        return NULL;
     }
   }
 };
 
 // CompileType describes type of the value produced by the definition.
 //
 // It captures the following properties:
 //    - whether value can potentially be null or it is definitely not null;
 //    - concrete class id of the value or kDynamicCid if unknown statically;
 //    - abstract super type of the value, concrete type of the value in runtime
@@ skipping 8113 matching lines @@
   LocationSummary* Name::MakeLocationSummary(Zone* zone, bool opt) const {    \
     UNIMPLEMENTED();                                                          \
     return NULL;                                                              \
   }                                                                           \
   void Name::EmitNativeCode(FlowGraphCompiler* compiler) { UNIMPLEMENTED(); }
 
 
 }  // namespace dart
 
 #endif  // VM_INTERMEDIATE_LANGUAGE_H_