Allow marking an overwrite as optional

src/main/java/org/spongepowered/asm/mixin/Overwrite.java

@@ -93,4 +93,13 @@
      */
     public boolean remap() default true;
 
+    /**
+     * By default, if the target for an overwrite cannot be found in the target class an
+     * exception will be thrown, however in cases where you do not want that behaviour
+     * you can mark it as optional, and it will silently not apply rather than throwing an exception.
+     * 
+     * @return True to not throw an exception if the target cannot be found
+     */
+    public boolean optional() default false;
+
 }

src/main/java/org/spongepowered/asm/mixin/transformer/MixinApplicatorStandard.java

@@ -481,7 +481,9 @@ protected void applyNormalMethod(MixinTargetContext mixin, MethodNode mixinMetho
      * @param method Method to merge
      */
     protected void mergeMethod(MixinTargetContext mixin, MethodNode method) {
-        boolean isOverwrite = Annotations.getVisible(method, Overwrite.class) != null;
+        AnnotationNode overwrite = Annotations.getVisible(method, Overwrite.class);
+        boolean isOverwrite = overwrite != null;
+        boolean isOptionalOverwrite = Annotations.<Boolean>getValue(overwrite, "optional", Boolean.FALSE);
         MethodNode target = this.findTargetMethod(method);
 
         if (target != null) {
@@ -504,7 +506,7 @@ protected void mergeMethod(MixinTargetContext mixin, MethodNode method) {
 
                 this.targetClass.methods.remove(target);
             }
-        } else if (isOverwrite) {
+        } else if (isOverwrite && !isOptionalOverwrite) {
             throw new InvalidMixinException(mixin, String.format("Overwrite target \"%s\" was not located in target class %s",
                     method.name, mixin.getTargetClassRef()));
         }