Fix bot breakage: math unary instruction has side effect and therefore cannot be eliminated

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/bigint_operations.h"
 #include "vm/bit_vector.h"
 #include "vm/dart_entry.h"
 #include "vm/flow_graph_allocator.h"
@@ skipping 1401 matching lines @@
       return false;
   }
 }
 
 
 Definition* ConstantInstr::Canonicalize(FlowGraph* flow_graph) {
   return HasUses() ? this : NULL;
 }
 
 
+// A math unary instruction has a side effect (exception
+// thrown) if the argument is not a number.
+// TODO(srdjan): eliminate if has no uses and input is guaranteed to be number.
 Definition* MathUnaryInstr::Canonicalize(FlowGraph* flow_graph) {
-  return HasUses() ? this : NULL;
+  return this;
 }
 
 
 Definition* LoadFieldInstr::Canonicalize(FlowGraph* flow_graph) {
   if (!HasUses()) return NULL;
   if (!IsImmutableLengthLoad()) return this;
 
   // For fixed length arrays if the array is the result of a known constructor
   // call we can replace the length load with the length argument passed to
   // the constructor.
@@ skipping 1708 matching lines @@
     case Token::kTRUNCDIV: return 0;
     case Token::kMOD: return 1;
     default: UNIMPLEMENTED(); return -1;
   }
 }
 
 
 #undef __
 
 }  // namespace dart