Fix ParameterMirror.hasDefaultValue/defaultValue in source mirrors.

sdk/lib/_internal/compiler/implementation/elements/modelx.dart

Index: sdk/lib/_internal/compiler/implementation/elements/modelx.dart
diff --git a/sdk/lib/_internal/compiler/implementation/elements/modelx.dart b/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
index b52189b9b4f7d7b51079b2cf7f30f92cb0b5d050..777e6a6392b289225d1a5b638ef20fda52435583 100644
--- a/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
+++ b/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
@@ -1169,6 +1169,20 @@ class VariableElementX extends ElementX implements VariableElement {
   Token position() => findMyName(variables.position());
 
   accept(ElementVisitor visitor) => visitor.visitVariableElement(this);
+
+  // TODO(johnniwinther): Move the patch/origin implementation to a parameter
+  // specific element when added.
+
+  /**
+   * A function declaration that should be parsed instead of the current one.
+   * The patch should be parsed as if it was in the current scope. Its
+   * signature must match this function's signature.
+   */
+  VariableElementX patch = null;
+  VariableElementX origin = null;
+
+  bool get isPatch => origin != null;
+  bool get isPatched => patch != null;
 }
 
 class FieldElementX extends VariableElementX implements FieldElement {