And stub the sponge classes so we can build

src/api/java/org/spongepowered/asm/mixin/Mixin.java

@@ -0,0 +1,5 @@
+package org.spongepowered.asm.mixin;
+
+public @interface Mixin {
+    Class<?> value();
+}

src/api/java/org/spongepowered/asm/mixin/gen/Accessor.java

@@ -0,0 +1,5 @@
+package org.spongepowered.asm.mixin.gen;
+
+public @interface Accessor {
+    String value() default "";
+}

src/api/java/org/spongepowered/asm/mixin/injection/At.java

@@ -0,0 +1,14 @@
+package org.spongepowered.asm.mixin.injection;
+
+public @interface At {
+    String value() default "";
+    String target() default "";
+    int ordinal() default 0;
+    boolean remap() default true;
+    Shift shift() default Shift.BEFORE;
+    
+    enum Shift {
+        BEFORE,
+        AFTER
+    }
+}

src/api/java/org/spongepowered/asm/mixin/injection/Inject.java

@@ -0,0 +1,7 @@
+package org.spongepowered.asm.mixin.injection;
+
+public @interface Inject {
+    String method() default "";
+    At at();
+    boolean cancellable() default false;
+}

src/api/java/org/spongepowered/asm/mixin/injection/ModifyArg.java

@@ -0,0 +1,7 @@
+package org.spongepowered.asm.mixin.injection;
+
+public @interface ModifyArg {
+    String method() default "";
+    At at();
+    int index();
+}

src/api/java/org/spongepowered/asm/mixin/injection/Redirect.java

@@ -0,0 +1,6 @@
+package org.spongepowered.asm.mixin.injection;
+
+public @interface Redirect {
+    String[] method() default "";
+    At at();
+}

src/api/java/org/spongepowered/asm/mixin/injection/callback/CallbackInfo.java

@@ -0,0 +1,5 @@
+package org.spongepowered.asm.mixin.injection.callback;
+
+public abstract class CallbackInfo {
+    public abstract void cancel();
+}

src/api/java/org/spongepowered/asm/mixin/injection/callback/CallbackInfoReturnable.java

@@ -0,0 +1,6 @@
+package org.spongepowered.asm.mixin.injection.callback;
+
+public abstract class CallbackInfoReturnable<T> {
+    public abstract T getReturnValue();
+    public abstract void setReturnValue(T t);
+}