Fix for issue 26243 - illegal motion of assignments in instruction merging

pkg/compiler/lib/src/ssa/nodes.dart

 // 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.
 
 import '../closure.dart';
 import '../common.dart';
 import '../compiler.dart' show Compiler;
 import '../constants/constant_system.dart';
 import '../constants/values.dart';
 import '../dart_types.dart';
@@ skipping 2247 matching lines @@
     sourceElement = variable;
   }
 
   toString() => 'local ${sourceElement.name}';
   accept(HVisitor visitor) => visitor.visitLocalValue(this);
 }
 
 class HParameterValue extends HLocalValue {
   HParameterValue(Entity variable, type) : super(variable, type);
 
+  // [HParameterValue]s are either the value of the parameter (in fully SSA
+  // converted code), or the mutable variable containing the value (in
+  // incompletely SSA converted code, e.g. methods containing exceptions).
+  bool usedAsVariable() {
+    for (HInstruction user in usedBy) {
+      if (user is HLocalGet) return true;
+      if (user is HLocalSet && user.local == this) return true;
+    }
+    return false;
+  }
+
   toString() => 'parameter ${sourceElement.name}';
   accept(HVisitor visitor) => visitor.visitParameterValue(this);
 }
 
 class HThis extends HParameterValue {
   HThis(ThisLocal element, TypeMask type) : super(element, type);
 
   ThisLocal get sourceElement => super.sourceElement;
 
   accept(HVisitor visitor) => visitor.visitThis(this);
@@ skipping 983 matching lines @@
 class HDynamicType extends HRuntimeType {
   HDynamicType(DynamicType dartType, TypeMask instructionType)
       : super(const <HInstruction>[], dartType, instructionType);
 
   accept(HVisitor visitor) => visitor.visitDynamicType(this);
 
   int typeCode() => HInstruction.DYNAMIC_TYPE_TYPECODE;
 
   bool typeEquals(HInstruction other) => other is HDynamicType;
 }