Clean patch implementation.

sdk/lib/_internal/compiler/implementation/ssa/builder.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.
 
 part of ssa;
 
 /**
  * A special element for the extra parameter taken by intercepted
  * methods. We need to implement [TypedElement.type] because our
  * optimizers may look at its declared type.
@@ skipping 1474 matching lines @@
           bodyElement = backendMember;
         }
       }
     });
     if (bodyElement == null) {
       bodyElement = new ConstructorBodyElementX(constructor);
       classElement.addBackendMember(bodyElement);
 
       if (constructor.isPatch) {
         // Create origin body element for patched constructors.
-        bodyElement.origin = new ConstructorBodyElementX(constructor.origin);
-        bodyElement.origin.patch = bodyElement;
+        ConstructorBodyElementX patch = bodyElement;
+        ConstructorBodyElementX origin =
+            new ConstructorBodyElementX(constructor.origin);
+        origin.applyPatch(patch);
         classElement.origin.addBackendMember(bodyElement.origin);
       }
     }
     assert(bodyElement.isGenerativeConstructorBody());
     return bodyElement;
   }
 
   HParameterValue addParameter(Element element, TypeMask type) {
     assert(inliningStack.isEmpty);
     HParameterValue result = new HParameterValue(element, type);
@@ skipping 4809 matching lines @@
     DartType unaliased = type.unalias(builder.compiler);
     if (unaliased is TypedefType) throw 'unable to unalias $type';
     unaliased.accept(this, builder);
   }
 
   void visitDynamicType(DynamicType type, SsaBuilder builder) {
     ClassElement cls = builder.compiler.findHelper('DynamicRuntimeType');
     builder.push(new HDynamicType(type, new TypeMask.exact(cls)));
   }
 }