Clean patch implementation.

sdk/lib/_internal/compiler/implementation/ssa/builder.dart

Index: sdk/lib/_internal/compiler/implementation/ssa/builder.dart
diff --git a/sdk/lib/_internal/compiler/implementation/ssa/builder.dart b/sdk/lib/_internal/compiler/implementation/ssa/builder.dart
index 954a95f8d7e4842dc9301da529ced66367e82356..efbad80568c103ba387cd8c888fd0f7aa24d8b21 100644
--- a/sdk/lib/_internal/compiler/implementation/ssa/builder.dart
+++ b/sdk/lib/_internal/compiler/implementation/ssa/builder.dart
@@ -1492,8 +1492,10 @@ class SsaBuilder extends ResolvedVisitor {
 
       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);
       }
     }