Fix for when key is a String rather than NSString. Otherwise you can get the following crash:

Could not cast value of type 'Swift.AnyHashable' (0x1027b3b10) to 'SwiftFoundation.NSObject' (0x101a56668).
2018-02-16 14:30:46.355774+0000 TestFoundation[7306:8715333] Could not cast value of type 'Swift.AnyHashable' (0x1027b3b10) to 'SwiftFoundation.NSObject' (0x101a56668).
(lldb) up
frame #9: 0x000000010165b114 SwiftFoundation`NSMutableDictionary.subscript.setter(newValue=some, key=Swift.AnyHashable @ 0x00007fff5fbfcd88, self=0x000060800e181fe0) at NSDictionary.swift:649
   646 	        }
   647 	        set {
   648 	            if let val = newValue {
-> 649 	                �[4ms�[0metObject(val, forKey: key)
   650 	            } else {
   651 	                removeObject(forKey: key)
   652 	            }
(lldb) up
frame #10: 0x00000001004c0443 TestFoundation`static Dictionary.twEncode(data=TestFoundation.TwoWayMirror @ 0x00007fff5fbfce00, self=[Key : Value]) at TwoWayMirror.swift:386
   383 	            #if os(Linux)
   384 	            let key = NSString(string: key as! String)
   385 	            #endif
-> 386 	            dict[key] �[4m=�[0m TwoWayMirror.encode(mirror: &mirror)
   387 	        }
   388 	        return dict
   389 	    }
(lldb)

Author: johnno1962

Foundation/NSDictionary.swift

@@ -586,7 +586,7 @@ open class NSMutableDictionary : NSDictionary {
         guard type(of: self) === NSDictionary.self || type(of: self) === NSMutableDictionary.self else {
             NSRequiresConcreteImplementation()
         }
-        _storage[(aKey as! NSObject)] = _SwiftValue.store(anObject)
+        _storage[_SwiftValue.store(aKey)] = _SwiftValue.store(anObject)
     }
 
     public convenience required init() {

TestFoundation/TestNSDictionary.swift

@@ -32,6 +32,7 @@ class TestNSDictionary : XCTestCase {
             ("test_mutableCopying", test_mutableCopying),
             ("test_writeToFile", test_writeToFile),
             ("test_initWithContentsOfFile", test_initWithContentsOfFile),
+            ("test_settingWithStringKey", test_settingWithStringKey),
         ]
     }
 
@@ -222,6 +223,12 @@ class TestNSDictionary : XCTestCase {
         }
     }
 
+    func test_settingWithStringKey() {
+        let dict = NSMutableDictionary()
+        // has crashed in the past
+        dict["stringKey"] = "value"
+    }
+
     private func createTestFile(_ path: String, _contents: Data) -> String? {
         let tempDir = NSTemporaryDirectory() + "TestFoundation_Playground_" + NSUUID().uuidString + "/"
         do {