Fix short jump address calculation in x64 disassembler

runtime/observatory/lib/src/service/object.dart

 // 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.
 
 part of service;
 
 /// Helper function for canceling a Future<StreamSubscription>.
 Future cancelFutureSubscription(
     Future<StreamSubscription> subscriptionFuture) async {
   if (subscriptionFuture != null) {
@@ skipping 3138 matching lines @@
 
   void _resolveJumpTarget(List<CodeInstruction> instructionsByAddressOffset,
                           int startAddress) {
     if (!_isJumpInstruction()) {
       return;
     }
     int address = _getJumpAddress();
     if (address == 0) {
       return;
     }
-
-    jumpTarget = instructionsByAddressOffset[address - startAddress];
+    var relativeAddress = address - startAddress;
+    if (relativeAddress < 0) {
+      Logger.root.warning('Bad address resolving jump target $relativeAddress');
+      return;
+    }
+    if (relativeAddress >= instructionsByAddressOffset.length) {
+      Logger.root.warning('Bad address resolving jump target $relativeAddress');
+      return;
+    }
+    jumpTarget = instructionsByAddressOffset[relativeAddress];
   }
 }
 
 class CodeKind {
   final _value;
   const CodeKind._internal(this._value);
   String toString() => '$_value';
   bool isSynthetic() => [Collected, Native, Tag].contains(this);
   bool isDart() => !isSynthetic();
   static CodeKind fromString(String s) {
@@ skipping 521 matching lines @@
     var v = list[i];
     if ((v is ObservableMap) && _isServiceMap(v)) {
       list[i] = owner.getFromMap(v);
     } else if (v is ObservableList) {
       _upgradeObservableList(v, owner);
     } else if (v is ObservableMap) {
       _upgradeObservableMap(v, owner);
     }
   }
 }