These warnings from the build give a hint:
javadoc: warning - Package java.awt not found
javadoc: warning - Package javax.annotation not found
javadoc: warning - Package javax.rmi not found
javadoc: warning - Package javax.transaction not found
4 warnings

And this page confirms it:
http://download.java.net/java/jdk9/docs/api/overview-frame.html

java.awt is missing from the docs bundle. The subpackages are present, but not java.awt itself.

$ ls jdk/src/java.desktop/share/classes/java/awt/*.java
jdk/src/java.desktop/share/classes/java/awt/ActiveEvent.java
jdk/src/java.desktop/share/classes/java/awt/Adjustable.java
jdk/src/java.desktop/share/classes/java/awt/AlphaComposite.java
jdk/src/java.desktop/share/classes/java/awt/AttributeValue.java
jdk/src/java.desktop/share/classes/java/awt/AWTError.java
jdk/src/java.desktop/share/classes/java/awt/AWTEvent.java
jdk/src/java.desktop/share/classes/java/awt/AWTEventMulticaster.java
jdk/src/java.desktop/share/classes/java/awt/AWTException.java
jdk/src/java.desktop/share/classes/java/awt/AWTKeyStroke.java
jdk/src/java.desktop/share/classes/java/awt/AWTPermission.java
jdk/src/java.desktop/share/classes/java/awt/BasicStroke.java
jdk/src/java.desktop/share/classes/java/awt/BorderLayout.java
jdk/src/java.desktop/share/classes/java/awt/BufferCapabilities.java
jdk/src/java.desktop/share/classes/java/awt/Button.java
jdk/src/java.desktop/share/classes/java/awt/Canvas.java
jdk/src/java.desktop/share/classes/java/awt/CardLayout.java
jdk/src/java.desktop/share/classes/java/awt/CheckboxGroup.java
jdk/src/java.desktop/share/classes/java/awt/Checkbox.java
jdk/src/java.desktop/share/classes/java/awt/CheckboxMenuItem.java
jdk/src/java.desktop/share/classes/java/awt/Choice.java
jdk/src/java.desktop/share/classes/java/awt/Color.java
jdk/src/java.desktop/share/classes/java/awt/ColorPaintContext.java
jdk/src/java.desktop/share/classes/java/awt/Component.java
jdk/src/java.desktop/share/classes/java/awt/ComponentOrientation.java
jdk/src/java.desktop/share/classes/java/awt/CompositeContext.java
jdk/src/java.desktop/share/classes/java/awt/Composite.java
jdk/src/java.desktop/share/classes/java/awt/Conditional.java
jdk/src/java.desktop/share/classes/java/awt/Container.java
jdk/src/java.desktop/share/classes/java/awt/ContainerOrderFocusTraversalPolicy.java
jdk/src/java.desktop/share/classes/java/awt/Cursor.java
jdk/src/java.desktop/share/classes/java/awt/DefaultFocusTraversalPolicy.java
jdk/src/java.desktop/share/classes/java/awt/DefaultKeyboardFocusManager.java
jdk/src/java.desktop/share/classes/java/awt/Desktop.java
jdk/src/java.desktop/share/classes/java/awt/Dialog.java
jdk/src/java.desktop/share/classes/java/awt/Dimension.java
jdk/src/java.desktop/share/classes/java/awt/DisplayMode.java
jdk/src/java.desktop/share/classes/java/awt/EventDispatchThread.java
jdk/src/java.desktop/share/classes/java/awt/EventFilter.java
jdk/src/java.desktop/share/classes/java/awt/Event.java
jdk/src/java.desktop/share/classes/java/awt/EventQueue.java
jdk/src/java.desktop/share/classes/java/awt/FileDialog.java
jdk/src/java.desktop/share/classes/java/awt/FlowLayout.java
jdk/src/java.desktop/share/classes/java/awt/FocusTraversalPolicy.java
jdk/src/java.desktop/share/classes/java/awt/FontFormatException.java
jdk/src/java.desktop/share/classes/java/awt/Font.java
jdk/src/java.desktop/share/classes/java/awt/FontMetrics.java
jdk/src/java.desktop/share/classes/java/awt/Frame.java
jdk/src/java.desktop/share/classes/java/awt/GradientPaintContext.java
jdk/src/java.desktop/share/classes/java/awt/GradientPaint.java
jdk/src/java.desktop/share/classes/java/awt/Graphics2D.java
jdk/src/java.desktop/share/classes/java/awt/GraphicsCallback.java
jdk/src/java.desktop/share/classes/java/awt/GraphicsConfigTemplate.java
jdk/src/java.desktop/share/classes/java/awt/GraphicsConfiguration.java
jdk/src/java.desktop/share/classes/java/awt/GraphicsDevice.java
jdk/src/java.desktop/share/classes/java/awt/GraphicsEnvironment.java
jdk/src/java.desktop/share/classes/java/awt/Graphics.java
jdk/src/java.desktop/share/classes/java/awt/GridBagConstraints.java
jdk/src/java.desktop/share/classes/java/awt/GridBagLayoutInfo.java
jdk/src/java.desktop/share/classes/java/awt/GridBagLayout.java
jdk/src/java.desktop/share/classes/java/awt/GridLayout.java
jdk/src/java.desktop/share/classes/java/awt/HeadlessException.java
jdk/src/java.desktop/share/classes/java/awt/IllegalComponentStateException.java
jdk/src/java.desktop/share/classes/java/awt/ImageCapabilities.java
jdk/src/java.desktop/share/classes/java/awt/Image.java
jdk/src/java.desktop/share/classes/java/awt/Insets.java
jdk/src/java.desktop/share/classes/java/awt/ItemSelectable.java
jdk/src/java.desktop/share/classes/java/awt/JobAttributes.java
jdk/src/java.desktop/share/classes/java/awt/KeyboardFocusManager.java
jdk/src/java.desktop/share/classes/java/awt/KeyEventDispatcher.java
jdk/src/java.desktop/share/classes/java/awt/KeyEventPostProcessor.java
jdk/src/java.desktop/share/classes/java/awt/Label.java
jdk/src/java.desktop/share/classes/java/awt/LayoutManager2.java
jdk/src/java.desktop/share/classes/java/awt/LayoutManager.java
jdk/src/java.desktop/share/classes/java/awt/LinearGradientPaintContext.java
jdk/src/java.desktop/share/classes/java/awt/LinearGradientPaint.java
jdk/src/java.desktop/share/classes/java/awt/List.java
jdk/src/java.desktop/share/classes/java/awt/MediaTracker.java
jdk/src/java.desktop/share/classes/java/awt/MenuBar.java
jdk/src/java.desktop/share/classes/java/awt/MenuComponent.java
jdk/src/java.desktop/share/classes/java/awt/MenuContainer.java
jdk/src/java.desktop/share/classes/java/awt/MenuItem.java
jdk/src/java.desktop/share/classes/java/awt/Menu.java
jdk/src/java.desktop/share/classes/java/awt/MenuShortcut.java
jdk/src/java.desktop/share/classes/java/awt/ModalEventFilter.java
jdk/src/java.desktop/share/classes/java/awt/MouseInfo.java
jdk/src/java.desktop/share/classes/java/awt/MultipleGradientPaintContext.java
jdk/src/java.desktop/share/classes/java/awt/MultipleGradientPaint.java
jdk/src/java.desktop/share/classes/java/awt/PageAttributes.java
jdk/src/java.desktop/share/classes/java/awt/PaintContext.java
jdk/src/java.desktop/share/classes/java/awt/Paint.java
jdk/src/java.desktop/share/classes/java/awt/Panel.java
jdk/src/java.desktop/share/classes/java/awt/PointerInfo.java
jdk/src/java.desktop/share/classes/java/awt/Point.java
jdk/src/java.desktop/share/classes/java/awt/Polygon.java
jdk/src/java.desktop/share/classes/java/awt/PopupMenu.java
jdk/src/java.desktop/share/classes/java/awt/PrintGraphics.java
jdk/src/java.desktop/share/classes/java/awt/PrintJob.java
jdk/src/java.desktop/share/classes/java/awt/RadialGradientPaintContext.java
jdk/src/java.desktop/share/classes/java/awt/RadialGradientPaint.java
jdk/src/java.desktop/share/classes/java/awt/Rectangle.java
jdk/src/java.desktop/share/classes/java/awt/RenderingHints.java
jdk/src/java.desktop/share/classes/java/awt/Robot.java
jdk/src/java.desktop/share/classes/java/awt/Scrollbar.java
jdk/src/java.desktop/share/classes/java/awt/ScrollPaneAdjustable.java
jdk/src/java.desktop/share/classes/java/awt/ScrollPane.java
jdk/src/java.desktop/share/classes/java/awt/SecondaryLoop.java
jdk/src/java.desktop/share/classes/java/awt/SentEvent.java
jdk/src/java.desktop/share/classes/java/awt/SequencedEvent.java
jdk/src/java.desktop/share/classes/java/awt/Shape.java
jdk/src/java.desktop/share/classes/java/awt/SplashScreen.java
jdk/src/java.desktop/share/classes/java/awt/Stroke.java
jdk/src/java.desktop/share/classes/java/awt/SystemColor.java
jdk/src/java.desktop/share/classes/java/awt/SystemTray.java
jdk/src/java.desktop/share/classes/java/awt/Taskbar.java
jdk/src/java.desktop/share/classes/java/awt/TextArea.java
jdk/src/java.desktop/share/classes/java/awt/TextComponent.java
jdk/src/java.desktop/share/classes/java/awt/TextField.java
jdk/src/java.desktop/share/classes/java/awt/TexturePaintContext.java
jdk/src/java.desktop/share/classes/java/awt/TexturePaint.java
jdk/src/java.desktop/share/classes/java/awt/Toolkit.java
jdk/src/java.desktop/share/classes/java/awt/Transparency.java
jdk/src/java.desktop/share/classes/java/awt/TrayIcon.java
jdk/src/java.desktop/share/classes/java/awt/WaitDispatchSupport.java
jdk/src/java.desktop/share/classes/java/awt/Window.java