Fixes to run "Hello, world!" on arm64 hardware.

runtime/vm/intermediate_language_arm64.cc

 // Copyright (c) 2014, 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_ARM64.
 #if defined(TARGET_ARCH_ARM64)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/dart_entry.h"
@@ skipping 71 matching lines @@
 // that will be overwritten by the patch instructions: a branch macro sequence.
 void ReturnInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   const Register result = locs()->in(0).reg();
   ASSERT(result == R0);
 #if defined(DEBUG)
   Label stack_ok;
   __ Comment("Stack Check");
   const intptr_t fp_sp_dist =
       (kFirstLocalSlotFromFp + 1 - compiler->StackSize()) * kWordSize;
   ASSERT(fp_sp_dist <= 0);
-  // UXTX 0 on a 64-bit register (FP) is a nop, but forces R31 to be
-  // interpreted as SP.
-  __ sub(R2, SP, Operand(FP, UXTX, 0));
+  __ sub(R2, SP, Operand(FP));
   __ CompareImmediate(R2, fp_sp_dist, PP);
   __ b(&stack_ok, EQ);
   __ hlt(0);
   __ Bind(&stack_ok);
 #endif
   __ LeaveDartFrame();
   __ ret();
 }
 
 
@@ skipping 5228 matching lines @@
   compiler->GenerateCall(token_pos(),
                          &label,
                          PcDescriptors::kOther,
                          locs());
   __ Drop(ArgumentCount());  // Discard arguments.
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_ARM64