VM: Initialize Instruction objects with break instructions instead of null.

runtime/vm/assembler_mips.h

Index: runtime/vm/assembler_mips.h
diff --git a/runtime/vm/assembler_mips.h b/runtime/vm/assembler_mips.h
index 4368707330b3ed36f6ab1f8bd52e5f098b4c24f7..714e2d8c923fef2d479cc739200c7c7603aa432d 100644
--- a/runtime/vm/assembler_mips.h
+++ b/runtime/vm/assembler_mips.h
@@ -504,11 +504,20 @@ class Assembler : public ValueObject {
     EmitBranchDelayNop();
   }
 
-  void break_(int32_t code) {
+  static int32_t BreakEncoding(int32_t code) {
     ASSERT(Utils::IsUint(20, code));
-    Emit(SPECIAL << kOpcodeShift |
-         code << kBreakCodeShift |
-         BREAK << kFunctionShift);
+    return SPECIAL << kOpcodeShift |
+           code << kBreakCodeShift |
+           BREAK << kFunctionShift;
+  }
+
+
+  void break_(int32_t code) {
+    Emit(BreakEncoding(code));
+  }
+
+  static uword GetBreakInstructionFiller() {
+    return BreakEncoding(0);
   }
 
   // FPU compare, always false.