Disallow multiple modifier keys

Trigger an error for incorrect use of `matchShortcut` by throwing an
error if multiple non-modifier keys are specified.

src/shared/shortcut.js

@@ -30,8 +30,10 @@ export function matchShortcut(event, shortcut) {
     const modifierFlag = modifiers[part];
     if (modifierFlag) {
       requiredModifiers |= modifierFlag;
-    } else {
+    } else if (requiredKey === null) {
       requiredKey = part;
+    } else {
+      throw new Error('Multiple non-modifier keys specified');
     }
   }
 

src/shared/test/shortcut-test.js

@@ -59,6 +59,12 @@ describe('shared/shortcut', () => {
         });
       });
     });
+
+    it('should throw an error if multiple non-modifier keys are specified', () => {
+      assert.throws(() => {
+        matchShortcut(createEvent('a'), 'a+b');
+      }, 'Multiple non-modifier keys specified');
+    });
   });
 
   describe('installShortcut', () => {