It appears that QT.Alt no longer works in QGIS 2.12. The correct reference is QT.Key_Alt. Enlightened by the link to the QT Key reference in this [answer][1].

The code below works with QGIS 2.12.1

    from functools import partial
    from qgis.utils import iface

    from PyQt4.QtCore import *
    from PyQt4.QtGui import *

    mappings = {"Layers": Qt.Key_Alt + Qt.Key_1,
                "Browser": Qt.Key_Alt + Qt.Key_2}
    shortcuts = []

    def activated(dock):
        dock = iface.mainWindow().findChild(QDockWidget, dock)
        visible = dock.isVisible()
        dock.setVisible(not visible)

    def bind():
        for dock, keys in mappings.iteritems():
            short = QShortcut(QKeySequence(keys), iface.mainWindow())
            short.setContext(Qt.ApplicationShortcut)
            short.activated.connect(partial(activated, dock))
            shortcuts.append(short)

    bind()

[1]: http://gis.stackexchange.com/a/168739/62159