Fix release x64 build:

runtime/vm/assembler_x64_test.cc

 // Copyright (c) 2012, 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"
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/assembler.h"
 #include "vm/os.h"
 #include "vm/unit_test.h"
@@ skipping 1008 matching lines @@
   __ movl(RAX, Immediate(0xfff0));
   __ andl(RCX, RAX);
   __ movl(RAX, Immediate(1));
   __ orl(RCX, RAX);
   __ movl(RAX, RCX);
   __ ret();
 }
 
 
 ASSEMBLER_TEST_RUN(Bitwise, test) {
-  uint64_t f1;
-  uint64_t f2;
+  uint64_t f1 = 0;
+  uint64_t f2 = 0;
   typedef int (*Bitwise)(void*, void*);
   int result = reinterpret_cast<Bitwise>(test->entry())(&f1, &f2);
   EXPECT_EQ(256 + 1, result);
   EXPECT_EQ(kMaxUint32, f1);
   EXPECT_EQ(kMaxUint32, f2);
 }
 
 
 ASSEMBLER_TEST_GENERATE(Bitwise64, assembler) {
   Label error;
@@ skipping 2415 matching lines @@
   EXPECT_EQ(ICData::kInt64RangeBit, range_of(Integer::New(kMaxInt64)));
   EXPECT_EQ(ICData::kInt64RangeBit, range_of(Integer::New(kMinInt64)));
 
   EXPECT_EQ(0, range_of(Bool::True().raw()));
 }
 
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64