Remove unchecked compile warnings

Removes compile warnings due to unchecked type conversion.

```
    [javac] /home/paul/dev/closure-compiler/src/com/google/javascript/jscomp/newtypes/PersistentMap.java:28: warning: [unchecked] unchecked conversion
    [javac]       EMPTY = ClojurePersistentHashMap.create(c);
    [javac]                                               ^
    [javac]   required: Class<? extends Map>
    [javac]   found:    Class
    [javac] /home/paul/dev/closure-compiler/src/com/google/javascript/jscomp/newtypes/PersistentMap.java:28: warning: [unchecked] unchecked method invocation: method create in class ClojurePersistentHashMap is applied to given types
    [javac]       EMPTY = ClojurePersistentHashMap.create(c);
    [javac]                                              ^
    [javac]   required: Class<? extends Map>
    [javac]   found: Class
    [javac] /home/paul/dev/closure-compiler/src/com/google/javascript/jscomp/newtypes/PersistentSet.java:29: warning: [unchecked] unchecked conversion
    [javac]       EMPTY = ClojurePersistentHashSet.create(c);
    [javac]                                               ^
    [javac]   required: Class<? extends Set>
    [javac]   found:    Class
    [javac] /home/paul/dev/closure-compiler/src/com/google/javascript/jscomp/newtypes/PersistentSet.java:29: warning: [unchecked] unchecked method invocation: method create in class ClojurePersistentHashSet is applied to given types
    [javac]       EMPTY = ClojurePersistentHashSet.create(c);
    [javac]                                              ^
    [javac]   required: Class<? extends Set>
    [javac]   found: Class
```