Cannot compile FXDnD with JDK 9: DropTargetContext::addNotify, removeNotify not found

As a result of the fix for JDK-8074763 we can no longer compile FX 9-dev using JDK 9 as the boot JDK. We currently use JDK 8u40 as our boot JDK, so our production builds are not affected. However, Swing / FX drag and drop will likely fail at runtime with a NoSuchMethodError as a result of this.

Here is the error:

$ java -version
java version "1.9.0-ea"
Java(TM) SE Runtime Environment (build 1.9.0-ea-b77)
Java HotSpot(TM) 64-Bit Server VM (build 1.9.0-ea-b77, mixed mode)

$ gradle
...
:swing:compileJava/localhome/kcr/javafx/9-kcr/jfx/rt/modules/swing/src/main/java/javafx/embed/swing/FXDnD.java:480: error: cannot find symbol
                    if (ctx != null) ctx.removeNotify();
                                        ^
  symbol: method removeNotify()
  location: variable ctx of type DropTargetContext
/localhome/kcr/javafx/9-kcr/jfx/rt/modules/swing/src/main/java/javafx/embed/swing/FXDnD.java:489: error: cannot find symbol
                        ctx.addNotify(FXDropTargetContextPeer.this);
                           ^
  symbol: method addNotify(FXDnD.FXDropTargetContextPeer)
  location: variable ctx of type DropTargetContext
/localhome/kcr/javafx/9-kcr/jfx/rt/modules/swing/src/main/java/javafx/embed/swing/FXDnD.java:511: error: cannot find symbol
                    if (ctx != null) ctx.removeNotify();
                                        ^
  symbol: method removeNotify()
  location: variable ctx of type DropTargetContext
/localhome/kcr/javafx/9-kcr/jfx/rt/modules/swing/src/main/java/javafx/embed/swing/FXDnD.java:520: error: cannot find symbol
                    if (ctx != null) ctx.removeNotify();
                                        ^
  symbol: method removeNotify()
  location: variable ctx of type DropTargetContext
Note: Some input files use or override a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
Note: Some input files use unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
4 errors
 FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':swing:compileJava'.
> Compilation failed with exit code 1; see the compiler error output for details
.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug
option to get more log output.

BUILD FAILED