Fix crash in modPow (issue 23693); various bigint knowledge missing.

runtime/vm/intermediate_language.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/intermediate_language.h"
 
 #include "vm/bit_vector.h"
 #include "vm/constant_propagator.h"
 #include "vm/cpu.h"
 #include "vm/dart_entry.h"
@@ skipping 1638 matching lines @@
 
 
 RawInteger* BinaryIntegerOpInstr::Evaluate(const Integer& left,
                                            const Integer& right) const {
   Integer& result = Integer::Handle();
 
   switch (op_kind()) {
     case Token::kTRUNCDIV:
     case Token::kMOD:
       // Check right value for zero.
-      if (right.AsInt64Value() == 0) {
+      if (right.IsSmi() && right.AsInt64Value() == 0) {
         break;  // Will throw.
       }
       // Fall through.
     case Token::kADD:
     case Token::kSUB:
     case Token::kMUL: {
       result = left.ArithmeticOp(op_kind(), right);
       break;
     }
     case Token::kSHL:
@@ skipping 1966 matching lines @@
     case Token::kTRUNCDIV: return 0;
     case Token::kMOD: return 1;
     default: UNIMPLEMENTED(); return -1;
   }
 }
 
 
 #undef __
 
 }  // namespace dart